For the purposes of section 315, the reference amount for a pre-commencement period is the amount in the pool at the end of the period—
(a)after the addition to the pool of any qualifying pre-commencement expenditure allocated to the pool for that period in accordance with section 316(3), but
(b)before determining, and adding to the pool, the amount of any pre-commencement supplement claimed in respect of the period.