I just hooked mine into an available vacuum port. My theory is that that vacuum will all level out eventually. The smaller ports will just take a little longer, in my opinion. I've seen people say that it should be around 25" while driving on the highway, but I feel that anything over 20" is just fine. elevation does play a role here too. The higher you are, the less air there is. The less air you have, the less your pump can pull through so the less vacuum you'll have. I've had a pump that would only make about 16.5" and I though the it may have been my cheap gauge. Then I had one the would make 22" so maybe the gauge isn't all that far off?