You could get a dual pressure gauge like the trucks use, red and green pointers on the same gauge... kinda like
http://images.google.com/imgres?img...firefox-a&rls=org.mozilla:en-US:official&sa=N
http://images.google.com/imgres?img...firefox-a&rls=org.mozilla:en-US:official&sa=N
I can't find the one I'm thinking of right now... it's in our second tanker. Want to say it's an ISSPRO, but an older model.
If you do some digging, you can find some pretty nice dual gauges, black faced for about $40 I think that Universal air suspension site had them.