@ Nautam, no spectrophotometric method can give good result when the concentration of P in your solution is more than 100 ppm. Better to dilute your sample so that P concentration does not exceed 50 ppm. I still feel chlorostannous reduced molubdophosphoric blue color method is the best for P estimation.

Dr. Trafalgar is correct in his recommendation. Any attempt to determine P beyond the upper limit of your standard curve will give unsatisfactory results. Thus you will need to keep your analytical sample concentration below 30 ppm (based on the upper limit of your standard curve). My recommendation is to do a 1:10 dilution which should get you somewhere in the middle of your standard curve for an actual concentration in your analytical sample.

There is no magic number for the dilution so you may have to experiment a bit to find one that will work for the majority of your samples. Once you find it, be consistent with its use (makes your calculations easier if you are running a large number of samples)

As far as the color development method, I don't have any real preference. I have used both methods and I an currently using the ascorbic acid method, mainly because it is the method commonly used by soil testing labs in the region of the US where I work.

http://soiltest.cfans.umn.edu/extractable-phosphorus