Household items/toiletries/cleaners expense and food share.
When one of the roommates purchases food or common household items like cleaners or paper towels or is constantly the one replacing these type of items; it would be nice to see a way to calculate their usage in order to properly pay for the necessities. I don't see that one person should be responsible for always buying these if they aren't the only one to use them. I had an issue a while back with a roommate not buying toilet paper because she said that she rarely was home and didn't think she used enough to really supply the household. I had a different opinion and thought it might be useful on splitwise.
We have a couple of ideas similar to this, but it sounds like the fundamental issue is your roommate disagreeing about usage. It doesn’t seem like asking them to track usage is going to be very successful. If you get them to agree to a fair share ahead of time, we have the ability to split by percent and shares, which should work in that case.