Use of Home Mortgage Interest

My understanding is that mortgage interest is an allowable expense in a Use of Home calculation (but not repayment).

My question arises because interest tends to be weighted at the beginning of the mortgage term so at the start this is likely to be a high figure and at the end a very low one.  Should the mortgage interest applied to calculation simply be the interest element as it shows on the mortgage statement or are we expected to find another way to arrive at the amount of mortgage interest?

With thanks in advance.