To obtain concentrations from fluorescence spectra, the fluorescence quantum efficiency as a function of wavelength of the emitting species has to be known, as well as the total absorbed light. Besides, unless you measure the total emitted light, a calibration of the geometrical distribution of the emitted light is required. The details may be read in a classical books on fluorescence like Principles of fluorescence spectroscopy by J.R.Lakowicz. 

However, if small concentrations and emissions are measured, from the equation

 IF=  FIF I0 (1-10^(-eps.c.l))

where FIF : fluorescence quantum efficiency at a given wavelength;,IF:  fluorescence intensity;I0: incident light intensity; eps: molar absorption coefficient at a given wavelength; c: molar concentration; l: optical path length of the cell,

 a lineal dependence of IFwith c may be obtained. In fact, for small c values  

 IF=  FIF I0 eps.c.l

and you may construct a calibration curve from the spectra of solutions of known (low) concentrations. If a straight line through the origin is obtained, you have chosen the conditions rightly!

In practice there is no clear cut method for chl fluorescence if you do not purify the chl a from b one, due to solvent and band overlapping. Suld you take in consideration absorption spectoscopy in any leaf crude extract as in Porra et al http://www.chem.ucla.edu/dept/Faculty/merchant/pdf/Porra_1989.pdfhttp://chemwiki.ucdavis.edu/Physical_Chemistry/Spectroscopy/Electronic_Spectroscopy/Fluorescence, http://omlc.org/spectra/PhotochemCAD/html/125.html

you can also use spectrophotometer, by calculating the wavelengths at 663 nm and 645 nm to calculate the chlorophyll content using the formula given by Arnon 1949

red fluorescence/far-red@ fluorescence