Good advice about ranging to other trees, especially if the ground is fairly steep. But if it is level then ther isn't going to be that much difference. Maybe a yard or two at 20, depending on how high you are.

The real trick to shooting from an elevation is to draw the bow as you would on the ground (level), anchor, and then bend at the waist to aim and make the shot. Too many guys just drop their bow arm down and this changes the whole upper body position, drawlength, and how the target is viewed through a peep. It might not seem so, but it is. That's why most shoot high and miss.

Think about it. If you range on a level and the target is 20 yards and then you range directly to the target on an angle it might range 22 yards. That might mean an inch or two difference. Guys who shoot high enough to miss are shooting a foot high. You figure it out. There is not a foot difference between 20 and 22 yards----even 25 yards.