If you don't know the history of the motor and what compression it had stock there's no way to judge health by compression alone. I've seen a cylinder 12 psi off the other 5 and have 40% leakdown.

Customer brought me a bass boat with a "FF" block 200 efi. Said it wouldn't plane. I took it to the ramp, planed fine, blew through the limiter to 6300 RPM's. I checked link and sync, timing, told him to come get it. He brings it back, says same problem. He checked compression and said they're all within 10. I repeat the check and the low one is 12 off the highest. Take it back to the ramp, planes fine, 6300 rpm, no problem. I tell him to come get it and meet me at the ramp with all the stuff he usually takes with him. Shows up with the boat loaded and a combined weight of about 700 lbs between him and the wife. I tell them to go out and see if it does it wile I watch from the dock. Sure enough won't plane. I hop on with them and it planes once I hang out over the bow. It's suddenly a dog. I take it back to the shop and start checking everything again. 12 psi difference between best and worst cylinder. Break out the Balzy. The 12 psi low cylinder has 40% leakdown. I recommend not to run it anymore and a rebuild. He runs it fishin a few more weeks and slings a rod through the block...

I do leakdown on EVERYTHING now...