We considered a system like that, but we soon discovered that we weren’t very good at the estimating part!

Everything that isn’t contributed to shared costs is ‘spending money’ for each individual, in our system: we still all have our “own money”. So for example: if our total mortgage repayments, bills, food shopping etc. for this month came to £2,000, of which I paid £1,200 from my salary of £1,500, and then at the end of the month the calculator says I should have paid only £800 (i.e. I “overpaid” by £400, and the others collectively “underpaid” by the same amount), then I’m owed £400 by the others. But regardless of anything else, the £600 I earned that wasn’t owed to the collective fund is ‘mine’ (and will probably be spent on, let’s face it, magic supplies, video games and booze).