Legalling splitting gas bill in Ferndale duplex

[David McCracken]

I currently own a duplex in SE Michigan, with a lower 2 bedroom flat and an upper 1 bedroom flat. There's only one gas meter. I've looked into splitting the ducting and plumbing for an additional furnace and hot water heater, but the estimates for that were far outside of my budget.

My two options are to either include the gas in the monthly rent by adjusting it accordingly, or to split the bill each month between the two tenants. I would prefer to split the bill each month between tenants, in order to hold them accountable for their gas usage. I would obviously be upfront about this and include an addendum to the lease regarding how gas would be billed. Can I write that addendum myself or should I seek legal counsel? Does anybody in Michigan have experience with this?