A key thing for both to remember is mortgage is the minimum amount the owner has to pay. Think new roof / repairs/ decorating / insurance etc. Rent is the maximum that you have to pay.

Also if the house was paid off with no mortgage still paying rent would be fair.

Personally my view is: c25% of FULL market rent. (Likely more than half the mortgage) 50% bills Joint Discretional spend (holidays, meal out etc) split as per post tax income ratios.

With house owner to pay for all household unexpected bills.

With a post marriage split to be more about having financial goals and evenish discretional spend (with highest earner having more to reward the hard work, and potential extra costs of more senior roles)