............ I’m figuring that once you raise the butterflies off idle by packing the throttle stop open a bit to achieve what corresponds to 3,000 rpm, any discrepancy in flow rate between the two will show as an increased pressure in the venturi on the air box side, due to reduced venting (flow), and the right can be brought back to the left by manipulation of the bell crank.............. .........
Exactly , as you know only the left side throttle body has a sacred throttle stop, the right side is simply cranked around to match. (The flow quantity doesn't need to be measured it's done by balancing the bodies)
So lets evaluate the left side throttle body what's it set to do? Flow a known amount of fluid with the butterfly at rest on the stop under Conditions A.
The fancy flow bench sets up the standard conditions to use for the measurement. lets call this Condition A and Quantity A
Now suppose you set up the same body under a different set of conditions, it would flow a different amount wouldn't it? Lets call this Conditions B and Quantity B (exact same throttle stop setting)
But by the same token you should be able to take another buggered up throttle body and subject it to Conditions B then adjust it to flow Quantity B.
If you then subject it to Conditions A shouldn't it flow Quantity A?
As I probably told you I'm an instrument technician, something we understand is how to measure stuff.
I propose we set Conditions B so that anyone anywhere can measure Quantity B with a few simple items from your nearby hardware store.