[vettestr]

The best bet is to follow the recommendation's of your component manufacturer. That being said .... are you looking for advise on checking the rocker tip to valve tip geometry? Several links on the web have very detailed instructions for checking push rod length that do a much better job than I could ever write.

I think you want more than just what is the ideal thread contact of a fastener but ???
A rule of thumb for fastener thread length is to match it's diameter or greater. If your bolt,screw or whatever is 5/16 in diameter then it should have that 5/16 of thread length as a minimum. In this case too long or longer than needed is just adding extra weight or mass that is traveling at 6000 plus crankshaft RPM.

The contact area of rocker tip to valve stem face is centered on the valve stem through the event. Seeing this area is tough but a light skin of grease on the stem face will show the contact at a given adjustment. I have made a few adjustable length pushrods copied from the old hemi days to make checking mock-up easier. Then measuring the pushrod to the thou. is done in stages as I don't have precision calipers or mic's beyond 6 inches. Jeff C