Nick,
I could hype one of my books where I detail this process, but cam or rocker-arm manufactures can supply this information. Of course, where you go for this info depends on whether you're building an engine with stock rocker arms or you're using aftermarket rockers.
Basically, the object is to have the rocker arm tip move equally from side-to-side of the valve-tip center and install the pushrod that makes this happen. Actually, I like the rocker arm contact the valve tip closer to the center at the fully opened valve position. Regardless, follow the manufacture's recommendations and you can't go wrong.
Check web sites for manufactures such as CompCams, Crane, Iskenderian T&D or Jesel for how to do determine rocker-arm geometry and pushrod length.
Tom
