If you have a GPS equipped airplane for a 3 degree descent: 3:1 rule. OK FL240 to descend to 10000 feet. You have 14000 feet to lose (+/- altimeter change muy importante) Divide alt to lose by 1000 = 14. Now take 14*3=42. You need to start down 42 nm out from your fix. To approximate a 3 degree glide path, use 1/2 your groundspeed * 10. So a 420 KTS G/S means 2100 fpm.
The VDP is based on the same principle. Say you need 1 1/2 SM to land. Using the 3:1 rule, you need 300'AGL for every 1SM of visibility. So for a 3 degree approach to landing you need to see the runway at 450'AGL in our example. Add 450 to 750, and your VDP will be at 1200 MSL.