How do you judge a hitter?
Neutral park - a hitter that is 60s across the board = .270 BA, .320 OBP and 20 HR?

IMO that's entirely dependent on the state of pitching in that world. 60s across the board is a gray area - could be good, could be disappointing, most likely streaky. I'd guess .270/.320/20 are at the higher end, though he might hit 20 HR easy. Again IMO, 70s are major league ratings, not 60s. Across the board helps but it means he's good at everything but not great at anything, so good pitching will handle him. My experience, having had a number of hitters who almost but never quite get to 70s, is that they almost but never quite hit like I wish they would.

I would say a decent place to start is 60's across means .600 ops, 70's .700, 80's high .800 etc. There are other factors like speed batting from which side and of course no one is likely 60's across the board. Look at other players and there ratings to get a feel.

I would add that not all ratings are equal. A player who is 60s across the board might not be as strong as a player with 40 in one and 70 in another, depending on which ratings are 40 and 70.

As brian and loud mentioned above, it is rare for a player to have consistent numbers across the board. Usually they excel in one area and have warts in another.

If I have to generalize, I would say:
60 across the board=AAAA player
65 across the board=major league player
70 across the board= potential occasional all star player
Anything more than the above, and you could have a HOF on your hands depending on playing time and injury through his career.