The simple math is that the stealer (if not in the SB) needs to succeed 58% of the time to show an immediate profit.

EV = (1.5sb * 58% win) + (-2sb * 42% lose) = +0.03sb



So, in order to thwart his steal attempts, you should be defending against him at least 60% of the time. Or sixty percent of his range. So, if he steals 50% of the time (or top 50% of hands) you should be defending with the top 30% (FWIW... A40 is boarder lining on the outside of 30% range).

Depending on how much better you play against him post flop, you can probably play more hands. But, for me, sticking around 60% or so, allows me to defend enough enough to spoil the steal attempts, yet gives me enough equity to make up for my positional disadvantage, IMO.

Just my 2 cents...