since you have a drop away you want to make sure the arrow is coming up BEFORE the tip passes the front of the riser. If not, you won't even be able to use a mechanical head without popping the arrow off the string. ( I learned this the hard way. My shop cut my arrows to short.........then i had to have the rest retimed, which put more pressure on my cables than i wanted........was a frustrating ordeal)
normally the arrow tip is 1.5 - 2 inches in front of the rest.
good luck