I'm still thinking about how to make this work in the context of your data structure.
In the meantime, I have two questions/concerns about the screenshot you provided. In your table, you are slicing by both Year/Month as well as Room Number. First, is there ever a scenario where an individual room number could have more days rented than available days in month? I'm seeing some of your cases where there are 37 Days Rented against 31 Available Days - I'm wondering if this is a bad count. Maybe this represents total number of rentals, rather than unique days on which rentals occured.
Second, by including room number in your visual, you might be inflating your total number of "Days Rented." For example, if two room numbers were both rented all 31 days, in your aggregation that will look like 62 Days Rented/ 31 Available Days. I'm thinking part of the solution to this might be removing Room Number from the table, and simply providing the ability to filter on this field.
Obviously, I might be misinterpreting your data and it might be a requirement to include that in the visual, so this might not work for you.
This looks like a measure totals problem. Very common. See the post about it here: https://community.powerbi.com/t5/DAX-Commands-and-Tips/Dealing-with-Measure-Totals/td-p/63376
Also, this Quick Measure, Measure Totals, The Final Word should get you what you need: