If the 30 yds you speak of is the straight line distance from you (elevated) to a deer (on the ground) then gravity acts on your arrow for less than 30 yds. The 30 yds would be the hypotenuse of a right triangle. The distance that gravity acts on your arrow would be the base of the right triangle (shorter than hypotenuse). Some simple trig calculations can help you determine the length of your triangle's base at any height and at any distance to target.

The best way to do it, IMO, is to just use a rangefinder to measure distance to some nearby trees while you are still on the ground. Then you don't have to worry about all the trig. But, since everyone shoot flat out to 40 yds anyways, the minor affect the angle of a treestand has on your shot is likely negligible.