I don't remember exactly where I grounded the Autometer gauge. I guess it doesn't matter anyway, since you say the only connection I'm concerned with is the signal to A-22. I have no idea why A-22 doesn't even need a ground.
I'm going to check this out tonight. To be able to log boost will be a huge help when tuning. I'd like to go out there right now and try this.
I'll post back with results.