Kathi,
ohhh, dear, you did have to ask that question! HA HA
I'll tell you what I told Steve R. We do this VERY scientific! We just dump some in!
Actually, in the gray and galley I don't use much, maybe half a cup, and I usually only use it when I am running so the agitation of the minimal liquid in the tanks will slush it around.
The black I add a bit more, plus some liquid laundry detergent and a VERY SMALL amount of bleach. Sometimes I don't bother with the bleach. This is again for running down the road with minimal amounts of fluid in the tank. When we are sitting, I may or may not use some Calgon ONLY, maybe half a cup.
I believe you might find some threads with more specific amounts posted, search the archives too! Seems it was quite a while ago that someone posted scientific amounts!
Good luck!