Calculating VDP was the original question...not calculating PDP (planned Descent point). If there is a VDP you will not need to calculate anything...it's published on the profile view of the non precision approach with a V symbol. There will be a DME amount beside the V so you will know when to descend. As far as what rate to descend, 95% of the time the VDP will coincide with a PAPI or VASI..so for 95% of the time nothing to calculate. For the other 5% of the time when there is not a visual glide path simply take half your ground speed and add a zero to detemine your vertical descent speed. (120 knots GS... 1/2 of 120 = 60...add a zero to 60 to get 600 feet per minute on your VSI). The previous answers were all correct for PDP which is the point on an approach where a pilot must begin the descent to the runway... but that point is not published like a VDP.
Hope that helps.
Hope that helps.