What kind of ammo are you using? Best way I know how to do it is to find out the velocity of the load you're using...if you don't know for certain, you can use an estimate for a generic 7.62x39 load. You'll also need the BC of the bullet. Again, if it's a factory load, that may be hard to find, but you can get an estimate by looking at MIdwayusa.com at some bullets of similar diameter, shape, and weight. Their website usually lists the BC for the bullets (in the reloading components section of their site). Then go to this site: http://www.hornady.com/ballistics/ballistics_calculator.php
and enter in the data. You'll also need the distance from your bore axis to the axis of your optic in inches. This site lets you enter in the range you have your rifle zeroed to, and then tells you where it will hit from 50 out to 500yds. Hope this helps. I like to take the data (two columns: range vs drop) and print a very small table which I then tape to the inside of my scope's flip up rear cover. Keeps me from forgetting when I switch between rifles/loads. Thus far this method has been very accurate for me. I did it with my 300WM, and the point of impact was within 0.5" of the estimate when I shot it at 300 yds for the 1st time.