Thanks to all. I suspect water-weight determinations should be done in a case fired in your rifle and before resizing (to keep the spent primer in to hold the water).
That said, let's assume a fired Remington case weighs 300gr and when filled with water, weighs 380gr, thereby having a case capacity of 80gr.
A Federal case may weigh 310gr but with water weighs 388gr. It therefore has a case capacity of 78gr.
In this case, whatever my load in the Remington case is, I should reduce by 2 grains in the Federal case.
Is that correct?
BTW, I appreciate the recommendation to use a fine powder. That would seem less sloppy but you need to be sure the "meniscus" is equal (the level at the top of the mouth of the case)