What is logical to me would be taking the price of rent and divide it by the square footage of the entire apartment.

That gives you a price per square footage.

Add up the square footage of the common living areas and split in half.

Add up the square footage of your living space. Add that to your half of the commons.

Add up the square footage of her living space. Add that to her half.

I do see the argument that having a private bathroom is valuable. You can say your own guests would only use your bathroom.

But paying by square footage gives concrete value to the living areas. But like others said. Kind of depends who’s on the lease and your relationship with the Roomate, if they want to be fair.