There is no direct equation as such in my view. However estimated levels or various zones of the marine environment can surely be deduced by P/B ratios.

Zivkovic and Babic (2003) Paleoceanographic implications of smaller benthic and planktonic foraminifera from the Eocene Pazin Basin (Coastal Dinarides, Croatia). Facies 49: 49-60

van der Zwann et al. (1990) The depth dependency of planktonic/benthic foraminiferal ratios: constraints and applications. Marine Geology 95: 1-16

Kroon et al. (1993) Planktonic and benthic foraminiferal abundances and their ratios (P/B) as expressions of middle-late Quaternary changes in watermass distribution and flow intensity on the northeastern Australian margin. In: McKenzie et al (Eds.) Proceedings of the ocean Drilling Program, Scientific Results 133: 181-188

Whatever paper suggest by Suman Sarkar will be effective for determination of paleodepth. I think work of van der Zwann et al. (1990) will help more in this respect. Also you may take help from benthic community to understand the same. Devide benthic foraminifera as shallow water (e.g. Ammonia, Bolivina, Elphidium, Nonionella etc.) and relatively deep water species. Then sea the variations. Also you may take help of the oxygen isotopic values of benthic foraminifera which also may guide you to understand variations in sea levels. Work of Miller et al., 2005, Science may help you. Please find the attached two papers.

What is the water depth and sea bed morphology ?. It is complex and needs to factor in various aspect like nature of sea bed sediment, distance from coast (organic flux),  identification of in-situ faunal assemblage, location of core wrt to river mouth and submarine active channels etc and of course time resolution-the sampling interval of the core.

Dear Syed

The environmental constrains of ocean systems control the spatial and temporal distribution of many planktic and benthic taxa so you will be able to calculate the sea level fluctuation by using planktonic and benthic foraminiferal abundance. The literature review of planktonic and benthonic foraminifera application described below.

Grimsdal and Vanmorkhoven (1955) were among the first to introduce the planktic/benthic foraminiferal ratio to determine the bathymetry of paleoenvironments (index of oceanity). Wright (1977) and Van Marle et al. (1987) studied recent foraminifera and were able to determine the basin depth using the percentage of planktonic foraminifera. Given that in deeper parts of the ocean (below 1600 m) foraminiferal tests are partly or fully dissolved, this method is not reliable for depths greater than 1600 m (Van Marle et al. 1987). Several studies (Berger and Diester Haas 1988; Gibson 1989; Nigam and Henriques 1992) demonstrated that the abundance of planktonic foraminifera increases with depth in the ocean.

Reference

1. WRIGHT, R. G. (1977): Planktonic-benthonic ratio in foraminifera as paleobathymetric tool. Quantitative evaluation; Annual American Association of Petroleum Geologists and Society of Economic Paleontologists and Mineralogists Convention (Washington, D.C.), p. 65 (Abstract).

2. VAN DER ZWAAN, G. J., JORISSEN, F. J., DE STIGTER, H. C. (1990): The depth-dependency of planktonic/benthic foraminiferal ratios; constraints and applications. - Marine Geology, 95; 1-16.

3. VAN MARLE, L. J., VAN HINTE, J. E. NEDERBRAGT, A. J. (1987): Plankton percentage of the foraminiferal fauna in seafloor samples from the Australian-lrian Jaya continental margin, Eastern Indonesia. - Marine Geology, 77; 151-156.

4. NIGAM, R., AND HENRIQUES, P. J. (1992): Planktonic percentage of foraminiferal fauna in surface sediments of the Arabian Sea (Indian Ocean) and a regional model for paleodepth determination. - Palaeogeography, Palaeoclimatology, Palaeoecology, 91; 89-98.

5. GRIMSDALE, T.F., VAN MORKHOVEN, F. P. C. M. (1955): The ratio between pelagic and benthonic foraminifera as a means of estimating depth of deposition of sedimentary rocks. - Proceedings of 4th World Petroleum Congress, Section I/D4, 473-491.

Yes, what is mentioned by KOMAR is a good attempt.

I have used the equation in this article. It is very efficient and you do not even need plankton.

Hohenegger, J.: Simple estimation of paleogradients based on presence/absence data: exemplified for paleodepth using benthic foraminifera, Palaeogeogr., Palaeoclimatol. Palaeoecol., 217, 2005.

With relatively little expertise in taxonomy of foraminifera, P/B ratios (corrected for endobenthics or not) can in principle provide an efficient tool. The depth conversion proposed by Van der Zwaan et al. (1990) indicated above is elegant as it provides a 'hard' number, although the realiability of this number is another issue, considering the range of variation in the data from the various basins included in that paper. A careful read of the paper prior to usage is a must, since it also outlines the various and severe pitfalls (e.g. downslope reworking, dissolution, anoxia) corrupting the method. 

The method of Van der Zwaan et al is based on a range of continental margin sequences and as such cannot be extrapolated beyond depths of about 1200 m. Actually, the formula provided gives a maximum range of 36 m to 1240 m (respectivly for 0 and 100% planktics). Not surprsingly, published reconstructed paleodepths by this method (e.g. Zivkovic and Babic, 2003 mentioned above) often approach the deeper limit. In other words, records from basins that were actually 1500 m or even 2000 m deep will always provide a reconstructed paleodepth of about 1200 m (98-100% planktics), as it cannot get any deeper with this method. The Arabian Sea is mostly to deep for the applicable range, but the margins are of course in the range of the method.

Finally and partly repeating from earlier comments, I'd like to stress that in many situations (and not only below the lysocline) foram assemblages are affected by some to severe dissolution. This decreases P/B ratios as planktics are generally more susceptible than benthics. A decrease by (artificial) dissolution from 99% to 98 % planktics may seem like little difference and is irrelevant for depth reconstructions. However, it actually means that the relative number of benthics doubles and if no benthics would have been dissolved it's easy to see that 50% of the planktics is actually lost! This does make a difference for other approaches using the composition of foraminiferal assemblages.

I think you may get advantages from the coiling direction sinistral or dextral  which reflect ,  cold an warm temperature .Golbigerna pachyderma may be good example.

 I think you can get also  advantages from the Size and coiling direction of Ammonia beccarri.  In addations top that you can get advantages from Gastropods species like  Amphidromus perversus

The key paper in which P/B ratios (excluding the endobenthics) are converted to paleodepth is by Van der Zwaan et al. 1990 in Marine Geology. In principle the conversion could be applicable to neritic and bathyal depths (max 1240m), but it has important limitations and uncertainties, notably the preservation of the assemblage (as planktic forams are generally more susceptible to dissolution than benthic forams). At bathyal depths it is not possible to distinguish minor depth differences of some tens of meters or less by this method. If you're not very experienced with foraminifera, don't use this method, as you will not be able to discern the many pitfalls. It could, however,  be interesting to compare the P/B based calculated water depth at the top of a piston core (=sea bed?) with the actual water depth.  

P% is the simplest way to express sea level geometry but not valid to estimate paleodepth, however, the benthic assemblages are well known methods for this measuring, particularly the upper depth limit of species.