Tony, You are not necessarily being cheated. I don't know how GM measures rocker ratio. But I worked as an engine development engineer at Ford and the way it was measured there was to publish the INSTANTANEOUS rocker arm ratio at the midpoint of the production cam's lift. That is also usually where the rocker arm contacts the valve tip at a right angle, giving the greatest mechanical advantage. So it is very likely that the lobe lift multiplied by the rocker ratio does not give the true lift at the valve. That's why light checking springs are handy for installing the valve train and measuring the true events of the cam, both duration AND lift. Good luck with your project.
By the way, aftermarket rocker arms I have tested usually will give greater lift than the production ones they replace, closer to the published ratio. Clever people, those cam grinders.
