Split it 6 ways. Each person is responsible for their share regardless of whether or not they live there. If they're claiming a space, then they need to cover for it.
Two people are set because they'll be living there.
Two people are covering for their share by getting sub-letters. (FYI - this is often illegal. You might want to check).
The last two need to cough up the rent and bill money or risk losing their rooms. They can: a) find subletters b) pay for it c) refuse to pay and risk having their rooms given away to someone else before September. After all, it's not fair to tie up two spaces and then not pay for them.
TRUST ME....you do NOT want the hassle of having to calculate some obnoxious formula only to have people argue with you about your math skills yadda yadda. It's irrelevant anyway...6 people renting = 6 people paying. Period.
.....Kelly
