Nice tweaked quote from a Boro lad there, one of Old Big 'Ead's finest.

But yeah, 11 points putting you on 91 guarantees you promotion mathematically from this point.
Millwall can get 90 max, so it rules them out full stop.
As others have said we have to play Ipswich. Therefore the possible maximum points totals of both sides depending on the outcome of that game are either;
Ipswich 93 Boro 89
Ipswich 91 Boro 90
Ipswich 90 Boro 92
In no eventuality do both those sides get to 91, so your getting 91 would 100% GUARANTEE you go up, even with an unfathomable GD shift in any direction.
FWIW I obviously don't think that will end up being needed for you to get promoted . The Back to the Future fan in me would like us to go up on 88

but regardless of my preference, it's most likely that we get anywhere in the region of 80 - 89 max, the lower figure representing our worst 7 game form of the season, the upper figure representing our best.
Right now I'd wager that 87 for you would probably 99.99% ascertain promotion and 89 in almost every eventuality hand you the title, especially considering your GD, though of course this may change as the games get ticked off.