Aren't all budgets set in advance though? Certainly all the ones I have seen have been, seems a bit counterintuitive to set a budget in arrears.
Quite. The algorithm for determining the budget was set in stone in advance. In fact Cameron even alludes to this in his speech. If he did not know this was coming then he is even more clueless than we all thought. The EU dont just dream up budgets on the fly.