You as a landlord may charge a cost-based rent that does not significantly exceed the sum of the capital cost and operating cost of the dwelling. The cost of capital is calculated as a reasonable annual return rate on the property market. An annual rate of return is reasonable if it is a couple of percent above the bank's benchmark interest rate. There is no link to your actual costs as the owner of the home, it is rather an estimated cost for the capital that is tied up in the home.
With operating costs means those costs, such as electricity, water, broadband and charge for a potential tenant. Is the home furnished you are allowed to claim reimbursement for furniture wear. Here it is your actual expenses that you can proceed from.
The tenant can have the rent level tested by the Rent Tribunal for future lease payments, but it cannot be charged retroactively.