On the gauge, I just got a air pressure gauge from AutoMeter that looked as close to the stock gauges as possible. I think it's 2-1/8" and just tapped into the hose and ran the line all the way back. I was able to get it backlighted red so it matches 'pretty good'.
The issue is however, that pressure is a relative thing. if you aren't on the bike, and pump it up, the bike don't begin to lift until you get up to around ~65 psi. then as it's raising, it stays at ~65 psi until you fully expand the shocks, and then the pressure will being rising above 65 psi. Again, I just pump the shocks up until the shocks without a rider fully extends. Then when you get on the bike, the reading will go up to about ~75 psi, and it will drop about an inch or so. I find that to be about the perfect setting for one up. If you add a passenger, you will notice the psi go up to around ~85 to 90 psi, and it will squat more. That's where I will pump it up some more to get the right balance. If while going down the road I hit a bump and bottom out, I'll just add about 2-3 seconds of air.
So, the gauge really doesn't give you a reading of relative height from low to high, but more what the actual psi is, and the height can be on the low side or high side with the same psi. It more is used for a status and a note point for where the psi starts rising again after the shocks fully extend. It helps you determine where this is, and keep an eye on things for piece of mind. I hope I explained this well enough?