i never could get a definitive answer to this question myself, so i tested it myself on 3 different rifles.
in my 1000 yard rifle (308), i found that meticulous brass prep made more of a difference than weighing cases. in fact, weighing cases proved to be a waste of time.
in my 7 mag, weighing cases did make a difference, but again, meticulous brass prep made more of a difference. the difference it made was barely noticeable from the bench, and certainly would not be discernable under field conditions.
in the 223, weighing brass did make a difference. enough of a difference that it is worth the time for that rifle.
overall, i agree w/ waksupi in that weighing brass isn't worth the time or effort, especially for rifles that won't be seeing long range competition.
if you do decide to weigh brass, make sure to start from a clean slate. new brass, trimmed to length so that all the lengths are identical. deburr the flash holes, and uniform primer pockets. now you can be sure that you are playing w/ an even field. then weigh your cases, and make your own cut-offs.
more direct to answer your question, that formula you have may work for 308-ish cases, but like you noted, the formula can't be universal because the weight of cases for alltogether different cartridges will be a lot different, and a .1 change in powder charge will affect a 223 tons more than it would affect a 7 rem mag...