Reasons to charge our tenant more than a third of rent and utilities?

My friend and I just purchased a home together. Went through a lot of pain getting the loan and everything else in order. A third friend is also living in the house with us but he is only renting, not purchasing the house with us. Now that we are settled in and ready to start living theres the question of what he is going to pay for rent. He thinks that it is fair that he pays an even split of what we are paying for the mortgage which is ridiculous because no landlord would ever have his tenants pay only what the mortgage on the house is. I can think of a ton of reasons but im sure there are more and obvious ones i don t know, cant think of. Can someone with experience this elaborate on why he should be paying more and maybe even a general idea of how much more he should be paying?

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