15 In Vacuum on Intake??

Are your durations 224/214 @ .050" lift? Probably so. If that is the case, 15" is decent for that cam. That assumes that your idle vacuum is at around 750-800 RPM; is that the case? And if you had a vacuum leak, you would be opening up the idle mixture screws MORE, not less. So it sounds all OK but let us know your full duration spec and idle RPM's.

We got 14-15" idle vacuum at 850 RPM on a 340 with a cam that has almost identical duration specs at .050" lift, and get 14.5-15 inches of vacuum. We had to fix up the PCV to the right type and adjust on the secondary idle stop screw and tighten up the secondary linkage to get there.