finding correct pushrod length

Shaft mounted rockers or stud mounted?
For stud mounted rockers I select a pushrod length that gives the best contact pattern. Don't know if that's the "right" way but that's how I do it.
A shaft mounted system pushrod length will not really affect the contact pattern as the pivot point is not changed with the change in pushrod length.
You need to move the shafts to adjust the contact pattern. Here I would select a pushrod length that gives you best "net" valve lift but ideally you would have the contact pattern sorted before hand.