I think a stronger theoretical defence for the BB facing a 2.5x open by the BTN has a more linear 3-betting range, leading to an overall strategy something like this:
Total playable range: 44.6%
CALL (blue): 32.9%
3-bet freq: 11.7%, comprised of:
3b/stack off (green): 2.1%
3b/flat a small 4-bet (pink): 2.9%
3b/fold (yellow): 6.7%
Clearly you can alter the strategy to exploit an opponent. Adjustments could include moving the pink hands into the stack off or 3b/folding ranges, or using more Kx and suited connectors as bluffs if villain folds "too often". Vs a nittier opener, you can/should also fold a lot more hands and just dump the worst blue hands (J4s, 32s, 52s, 87o, Q9o, A6o etc).
Note that facing a 3x open, the strategy would change significantly, as a lot of the offsuit aces and worst offsuit and suited hands become folds, and you should be more inclined to flat with hands like 99/88, AQo, A9s, A8s, as villain's opening range and continuance ranges are stronger (he doesn't flat the 3-bet with as many dominated hands).
EDIT: I should probably have put 85s into the flatting range in preference to 52s. There are a lot of debatable hands at the bottom of the calling range. e.g. It might be argued that A2o or T8o are slightly better than J4s.