Hey Chris,

It's not really a matter of how far you screw down the nut, it's about where the roller sits on the top of the valve stem. You should screw down the rocker so that the roller is positioned approximately 1/3 of the distance from the center of the valve stem on the intake side. But more importantly, you want the roller to have the least amount of travel (from end to end) as possible as you want the most pressure to be in the center of the stem to prevent guide wear. Here's a nice article to follow for checking pushrod length. It should answer all of your questions about proper pushrod length and how best to measure...

http://www.compcams.com/Technical/Products/Pushrods/