I have a Chevy 350 CID 4 Bolt Main 400 HP with a 750#4160 vac secondaries Holly Carb. I have adjusted the idle mixture screws to peak vacuum of 17" of vacuum at idle. I have 4" of vacuum at peak RPM (4200 RPM).

What should the vacuum be at peak RPM?

Thank you in advance.  

whats the reason for the question or are you just curious?  

I need to know how much vacuum is needed to pull off the secondary vacuum diaphragm. (I do know about the different strength of springs that can be installed in the vacuum pot) This is because my secondaries at wide open throttle are barley opening. Is 4" of vacuum enough vacuum to overcome any spring in the system?  

The only vaccum equation thats definate with a Holley is idle vac. , to chose the correct power valve. Holley makes a spring kit to custom tailor secondary opening. Unless someone was already in there, you should have the medium spring(cant remember the color). Keep switching to lighter spring till you hit gold. 4" vaccum at full throttle should overcome the light spring.  

4" is quite a lot for a performance ride, as any vacuum at WOT indicates a restriction. Just as a fun fact NHRA Pro Stock motors will probably have .1-.3 in. of vacuum at WOT. Overall, you have plenty to crack open the secondaries.

Secondaries aren't likely to come open when there is no load on the engine (like as in revving in neutral with someone looking at the carb). The best way to tell if your secondaries are opening fully is to take the vehicle out for a WOT stomp, wire the secondaries shut, and take out again and see if the performance is significantly worse.

The wrong vacuum diaphram may also cause your secondaries not to open.