The best spot I've heard is the large vac line running across the firewall on top from the intake near the TB, to the brake booster, tap as close to the TB as you can for better accuracy and avoid the valve closer to the booster. If you havent found a way to tap thatt vac line yet, I am working on a design to print that provides a coupler and stem (somewhat of a Tee) that can go inline with the large vac line and a regular vac hose can be routed out of it to the gauge. I rigged a Tee inline with mine with limited equipment since the size difference doesnt exisit in a Tee that I could find, I put in a Tee for the size I needed to run to the gauge and clamped a hose over the top of the line I tapped into. It's jenky to say the least so I am developing something a little cleaner and purpose built. Ill make a write up on it when I have it made