Holley is correct, it's 1/2 idle vacuum in gear (for automatic) or in neutral for stick. On a stand you can't get into gear but if it's at 11hg manifold now....it'll drop in gear. 4.5 is my guess.
Also, you increased the accelerator pump size which is rarely done except double pumpers mostly. Not saying it's wrong but I think you're over fueled from both power valve and accel pump shot. You might have the idle mxture screws misadjusted?
The power valve should be closed at idle and open when you accelerate (when vacuum drops even more). At idle it's NOT in the idle circuit.
None of this might be causing your backfire though. If the backfires clears out I think the existing power valve will be an issue later.