I'm certainly no expert in carb's, but to clarify, you are not simply revv'ing the engine to various rpm's, you are driving and putting the engine under load to said rpm's correct? As I'm pretty sure revv'ing an unloaded engine to various rpm's in neutral will not open vacuum secondaries.
To follow up with that, you are really testing two systems, the "when it opens" and the "rate it opens". The "when" is vacuum dependent, and on anything other than Q-jet's I'm not sure how to change the "rate".
On my old 400 powered Firebird and my Centurion's Buick 455, both engines pull(ed) over 15inHg at idle and I tuned the Q-jet secondaries to not open until 5inHg. On engines that pull decent vacuum, my opinion 5inHg is a good starting point for balancing economy versus power. That way the secondaries weren't opening on every hill I encountered. At least for 6.6L+ engines =P
