coelk, dont shoot in that kind of weather, Its not worth the powder and lead you waste.I cant even get accuracy out of my flintlocks or sidelocks with that kind of heat. I always do my shooting around 630 when i do 75+ yards. Your powder charge seems right, but i'd give pyrodex RS a try and set it at 90 grains.
How do you like the rifle so far? I really like the xlt.