I've only had to alter a load once due to lot variations since starting to load in 1963. That was in the late 1970's, when changing over to a new keg of Winchester 748. For that keg, and that one only, I had to drop my load for .223 one full grain.
I usually try to buy my powder by at least the 8 pound keg, and usually by the 32 pound case, just so the lot will be consistant for the greatest number of loads.
Hope this helps.
Fred