Every tank/system will be different. Sure you can discuss generalities, but it's pretty easy to find your baseline daily consumption. Test your water and record your findings on Day 1. Do not dose anything for a couple of days and then do another set of tests.
What ever the difference is measured is your consumption for the number of days you did not dose. So if calcium started at 450 on Day 1 and is 400 on Day 3, your daily consumption is 25, and you can dose accordingly.
My $.02 anyway.