Hrm, so it is.
Did some more testing, and it seems they were going after Jaheira specifically. Because when making it so that available targets was just DD and Minsc, with DD closer target, they did not switch targets, at either the base 10 resistance or buffed 60 resistance. Bringing Jaheira back in so that it was a line of DD, Minsc, Jaheira, they would skip to shooting at Jaheira.
So I suppose it was Jaheira being part caster that was giving her priority.
I then edited to have DD with 40 base resistance, put DD closer with Minsc, again they shot DD until he put up Def Stance, giving 90 resist, at which point they switched to Minsc. Actually behavior got wierd then. After getting shot a couple times, I pulled Minsc back out of sight, and they wandered a bit getting out of sight of DD around corners, then when I chased them down with DD they switched from crossbow to axe and then engaged melee.
Alright, thanks.
Frankly, I consider 75 too high even, that means someone who is effectively 4 times tougher. I was considering 60% because that would be 2.5 times the effective life.
Edit-I should clarify that melee behavior seemed to make it hard to say who was closer when retested, I could flip it between Minsc and DD at the drop of a hat by moving around the melee duergar. That was done at base 10 resist, I consider the ranged tests more important.