Best/fairest way to split Utilities in a House + ADU lot

[Francisco Franco]

Hey BP Family,

I finally purchased my first property and we are excited to meet our goals. The lot has a main 3/2 House with a detached garage and above the garage you guessed it there is a 1x1 Studio. Main house has all appliances in the house, Studio has separate W/D in the garage. We purchased the property already rented and working with the House and ADU occupied. There is only single meters for the property. The previous owner had a system/agreement in place with tenants that I took over and continue to make things easy. Studio paid $60/m extra for Utilities and House paid the rest. The studio lease is up next month and the tenants are leaving, I have the opportunity to make changes and with the main house on a 2 year lease things look good.

That is the plot here is the issue I want to ask all of you for your expert advice: Main house tenant is claiming that $60 all inclusive for studio's utilities is not very fair. They claim they don't use water or turn on their AC that much. They asked me to make it more fair. I am all about being fair and I can see their point. I have no back bills or utility records to make a baseline for what is fair. 

Should I wait a few more months to have bill history to make a baseline decision? Should I do a percentage split instead of a set price, maybe a 60/40%? Or just raise the Studio's Utilities?

I don't want to spend any out of pocket money at this point, so all options have to be based on this.

Thank you for your time and I look forward to reading everyone's experience,

Franco