Between 400-500psi depending on altitude. I like to see 450 as an average, but it'll vary somewhat with altitude and engine condition. As long as you are over 400 it'll still run OK, when you get cyls dipping into the 300's then it's time to start considering an overhaul.
You can get a compressor guage form places like Snap-On, Cornwell, Matco and OTC. There are several other places that sell the kits such as Diesel Innovations and IIRC ITP does as well. You can also make your own if you have an extra glowplug laying around. Gut it and braze an 1/4" quick-connect air fitting on it and plug it into your compression guage.