The chevy performance site has got some great tech articles on this topic:
Rocker Efficiency
Ratio Effects

It's important to keep in mind that fixed fulcrum rockers behave differently then the sloted ones:


Fixed fulcrum rockers (216, 235, 261, GMC 270 & 302) have tendency to fade as they lift since the rocker face is actually receding from the valve stem as traverses its arc. Stamped slotted rockers exhibit the opposite effect, with rocker ratio increasing at higher lifts. Many roller rocker setups claim to be constant ratio.

In the end the only way to tell what is going on with a particular package is to put a dial gauge on the valve and actuate the valve train.

regards,
stock49

[Linked Image from 49fastback.com]