You should be fine with 70 grains of powder out to 100 yards or so. If that's what gives you the best accuracy, then that's what I would go with. But if you want to switch to 80 grains, it should not change the point of impact of your bullet very much, especially at close range. I've gone from 70 grains to 100 grains loose powder, and it only changed the point of impact by about 1" at 50 yards and 4" or so at 100 yards.