I too was wondering about the correct pressure on the 1986 GL1200 Interstate because the book simply says 0 - 6 psi for the front, and 28 - 57 psi for the rear. But because I am a little short, even the max pressure forces me to tip-toe at a stop and that's with boots on. So I set mine around 15 psi; just enough air pressure to ensure oil in the strut doesn't back-flow into the service hose. I have to rebuild mine anyways, so at this point, due to it leaking oil (left side rear), the pressures do not concern me at the moment. Once I either replace or rebuild both struts (might as well do them both), then I will be meticulous about the correct air pressure for my weight. I wonder if they make a seat that is at least 1/2" inch lower.
I run 45lbs in the Rear, 4lbs in the front I have Progressive springs as well. Im 260lbs Very seldom 2up. I like just enough that when I jump off the center stand, I can feel the air in the rear. if you do it a few times you will feel what Im saying. With these settings I dont get much road feel, and the grooved pavement is pretty much unnoticeable. You have to use a quick release valve to get these things just right, because they dont hold much so pressure changes quickly. A small handheld pump will work great, then you dont go over pressure.