Splitting shared laundry costs

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Hi all

I have a vacant triplex (4/2/1 beds) that I am getting ready for rent. During the repairs/renos I added hookups for stacking laundry in each unit but I don't have the budget to buy individual machines right now. I also added a hookup in the common area. 

My plan is to buy a washer and dryer for the tenants to share until I can get them individual ones. Prepay coin/card machines aren't an option. The house main and hot water is on a separate bill so I need to split it between the tenants. I know for sure there will be complaints about who uses it less or more so the percent of the bill has to be agreed in writing before anyone moves in.

My question is how should I split the cost in the lease agreements... per bedroom, person listed on the agreement, another way? What are your stories and lessons from dealing with this type of situation?