Proper arrow length is one inch in front of where you would measure your brace height from, or the deepest part of your grip. It should be about 3/4 of an inch less than your draw length. Since the arrow charts do not ask for your draw length they calculate it from your arrow length. If you don't tell the chart or program that you are using an overdraw it will calculate your draw length wrong. One inch probably will not make that big of a difference, but two or three may get you the wrong arrow if you don't compensate for it.

Most prong rests should be set up right above your grip, and the arrow should be 1 inch past where the arrow contacts the prongs. Some of the drop aways have a slight amount of overdraw built into them. Any of the ones that require the rest arm to be behind the shelf will be anywhere from 1 to 3 inches of overdraw, like a muzzy zero effect, drop zone or mirage. However with these rests you don't have to shoot a shorter arrow. You can still shoot the correct length arrow for your draw if you want. It shouldn't make a huge difference. A lot of people shoot the same length arrow as thier draw length, that works ok in most instances as well. Unless you pick an arrow that is really close to being weak in the first place.

Download eastons tuning guide, lots of good info in it.

Good luck,
Paul