Every engine combination will have a slightly different "optimal" vacuum level. The vacuum at WOT is the most important and the idle vacuum will depend on the "health" of the pump and the pulley sizes/ratios and it all depends on the engine having a good seal. This is a timely question as I am installing a Moroso "enhanced" 4 vane on Project Muscrate and I will cover the install and tuning in a soon to follow article. As a good starting point I would shoot for 12 inches at WOT and IF you think you need more inch you're way up from there. Anything over 15 inches is asking for trouble unless you're engine has been specifically built to use a high amount of vacuum. Wrist pin clearance is a major issue with very high vacuum readings.
Hope this helps.
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