For those long shots, you would be better off using #5 shot, rather than #6. The pellet energy of #6 is right at the edge of effectiveness, which your two birds have show. Inside 40 yards, they work. At 40 yards and beyond, its iffy. #5 will work out to 45 yards at least, and perhaps out to 50 yds.
I think you will find that 80 grains of powder will open your pattern. Stick with the 70 grain load you are using. Use #5 to kill birds that far in the future.
As to range estimates, if you are shooting on familiar ground, take a string and stake and measure off 40 yds in a variety of directions and either mark a tree with a blaze, or plant a stake you can see from where you sit. That way you will know your yardage, and not have to do that by guess and by golly stuff we have all done before. :hmm: :youcrazy: :rotf:
Congratulations on two fine birds. Good hunting.