In challenge mode, you can 'get away' with 65kish health and not still get one shot, granted your dmg resist is 62%+. You should be able to mitigate ONE 'out in the open' shotgun body shot. But, only one without sig or smart/mobile cover. Pretty sure sub 60k results in one shots. Having 75-80k is great to help with a myriad of other dmg that comes your way, but if you're reworking all your stats just to get the HP, know that you'll get 2 shot wrecked by a shotgunner if you are somewhere in the 65-100k mark no matter what. This is the reason why HP in challenge mode isn't as important as most think. You will get 2 shot if you're low on the HP totem poll or if you're at a tanky 100k. Still 'try' to eventually hit that 75k mark, there's a million other reasons to have higher health. But just for the challenge incursion, you can cheat by 10-15k and not have it make much of a difference. Just don't go below 60ish K ever. Just something to think about.