Well I think perhaps it's an if() problem.
I have 3 columns, A, B and C labeled “IN”, “OUT” and “BALANCE” in row 1.
So in row 2 I feed last months carry over balance into “2A” and set “2C = 2A”
Then row 3 onwards I set “3C = 2C + 3A – 3B”
(new balance = previous balance plus IN minus OUT)
But...