Suggested formulas for RUBS

[Angie Y.]

Hi all! I have a property that I'm renting where the gas and water utilities can't be split out. One property is a 1BR/1BA 450 sq ft unit with one tenant, and another is a 4BR/2BA 1450 sq ft unit with 4 tenants. I'm going to implement RUBS, but am looking for guidance on the proper allocation here. 

Both stoves/oven are gas, and the dryer in the main house is gas. There is no yard to water, just regular usage with bathrooms, dishwasher and washers. 

Would I want to do this by square footage or occupant? Any suggested formulas to use? Thank you!