I run a water fund for my office, and I'm trying to fairly account for people
who leave the office for business and don't use the water for anywhere from a
week to a month.

Currently, I'm entering the invoice total in one cell, and in another
dividing it by the total participants minus a countif function (I mark an "X"
by their name for the month if they are out for a significant amount of time).

This is a little clunky, though, and I'd like to account for people who are
out 1-3 weeks by using something like a .25-.75 system, so nobody pays too
much for water they don't use or pay for someone else.

Any suggestions?

Thank you