This is really so dependent on the situation, villain and the games you are playing. I don't think you can definitively say a certain percent is too high or too low. I'll give a few thoughts on flatting though.

In position:

-You should be less likely to flat middling hands ip if there are aggressive squeezers behind you in the blinds.

-Against somebody who calls too many 3 bets I will widen my 3 bet value range and narrow my calling range preflop.

- Against somebody who folds to too many 3 bets I will 3 bet sc's, gappers and air more and flat instead of 3 bet broadways that can make dominating pairs.

- Against an aggressive 4 better I would widen my 5 bet shove value range which would also polorize my 3 betting range. I would not 3 bet something like AJ or KQ unless I was calling a shove. This all depends on villain.

-When evaluating what to do to an early position raise you should consider how wide they are opening and adjust accordingly. I don't tend to play a wide range vs ep open's regardless of position.

OOP

I don't flat very much oop unless villain is bad or likely to pay me off. Then I can widen my oop calling range a bit but I still wouldn't play too many hands oop without initiative. Your SB range should be smaller than your bb calling range.

In general I think having a tighter calling range allows you to play that whole range more aggressively and you can credibly represent a wider range of hands than you are calling with.


Feel free to add to this or ask any questions if I need to clarify one part of it.