n00b Question - Private Spreadsheet, Trouble working out what to use to get required value

Sorry for the ambiguous title, I'm working on a spreadsheet for my housemates that basically works out what everyone owes based on the amount of time they owe for, we have this sh*tty system where if someone goes away for a week or two they don't have to pay for the rent they weren't here for... anyway, so far I've got everything bar working out how much people owe based on the remaining balance, I had a solution but it would only really work for one specific person and assuming there would only be one person that isn't paying the full amount, but unfortunately there's been times where there's been more than one person =\ Your help would be greatly appreciated, I just can't figure out what I should use to do it cheers

Monthly Rent.xlsx