RUBS Calculations - Looking for Formulas!

[Bruce Gardner]

Does anyone have or use a pre-existing "calculator" for figuring out monthly utility charges for tenants using RUBS (Ratio Utility Billing System)?

I'm an expert level Excel guy, but the "logic" behind how to calculate the charges is getting complicated for a 6-unit I own.  It's easy if it's fully occupied, I can just divide the bill by the portion of sq. ft. used by each unit (they're not all the same).  However, as soon as I get one or more vacancy, it gets complicated.