To me old priest (circa Unleashed in the East) is "medium" gain. "low gain would be Live & Dangerous era Lizzy. "high gain" to me is something along the lines of John Sykes MESA tones on Whitesnake and Blue Murder...and is as high gain as I like generally.
I'm with Erik...the less gain you can get away with, the better...higher gain makes things easier to play, but sounds worse generally IMO...you lose the character of the wood, amp, pickups, etc. It gets to where you cannot tell a Superstrat from a Les Paul, and then what's the point?
My sound is somewhere between that unleashed tone and John Sykes. It's very much a cranked old metalface/jmp 2203 level of gain..use a goose for leads. Frank Hannon's or Adrian Smith's tone circa 1983-1985 is what I go for generally. By todays standards that's probably "medium" gain.