I've asked a lot of times, how much deviation is acceptable? We all know that different weights of powder result in different FPS but at what point does any of it really matter to small group or hitting the dot in score shooting, 2 FPS, 12 FPS, 20 FPS? Everything matters so the trick is to figure out which ones are worth spending a lot of time and money on, in my view. I don't know if Jack throws charges still but if he is then he must have decided at some point he didn't need exact powder charges to make tiny holes and the same is true for primer weights and thickness.
Me, for instance, years ago I decided cartridge cases were simply a container to hold the things that really matter. With all the testing I have done throughout the years, It has always been Powder charge and seating depth that made the accuracy. I still feel that way so I am fanatical about exact weight of powder and exact overall length of loaded rounds. My testing has always told me I was on the right road.