Rental Income - How to calculate interest

Hi, i have rented out my apartment in June this year and therefore i believe i will have to pay my first tax by end October 2013.
I know i can offset 75% of interest paid against my taxable income to calculate my tax due.
My question is how do I calculate this? Do i need my bank to confirm the exact interest between June 1st and December 31st or can I do this myself. Interest is posted to my mortgage account on a quarterly basis on the 15th of each month and not too sure what exact period it relates to.
Is it ok to estimate this or should the bank give me something in writing if I was audited?

You should get an interest cert for Jan-Dec 2012.
You'll take 7/12 of this and allow 75% of it as your interest expense.
That'll be fine in the event of an audit or query from Revenue.