I guess I am not following you.
When rebuilding/modifying an engine there are several factors that can alter rocker-arm geometry which in turn drives push-rod length. Milling the head, sinking the valves, not compensating with shims under the rocker perches, zero decking the block all add to changes from stock length.
Next comes the variables of a reground cam with a smaller base circle and higher lift. Which is why most mock-up assembly and measure for length.
Is there a measurement you are perhaps already shooting for?
