All that matters is that you have a winning record and at least a 50-55 RPI. Then, you are in. If you don't meet both those criteria, you are probably out.
I would say top 40 is fairly safe in the 68-team field.
To say top 50-55, that woudl mean 13-18 teams get in the tournament based on automatic-qualifying only. It's usually on the higher end of that. Usually teams seeded 13-16 would not be in the tourney if not for the auto bids, so that's 18. But I'd say that 40-50 range is more of a bubble range typically.