How do you calculate your tax deduction on your Interest payments?

Hello everyone,

Sorry but I'm just not sure if my calculations are right. and wondering if somebody could please help.

I have just settled on my first IP and with the first Interest only payment due at the end of the month I'm trying to work out a rough figure with the shortfall I'll be required to service the loan. But I get confused when I minus the Tax deduction on Interest payments. I was planning on using the tax variation form so each pay fortnightly.

My marginal rate is 30% (about 60K pa)

Do I minus the 30% from the annual Interest Only payments which are about $ 24,600

or do I minus the 30% after I have taken the the yearly rental income from the Interest payments?

e.g $24,500 – $13,000 = $ 11500

Because on both calculations I get different rough shortfalls after the deduction?

I can't seem to work out an approx shortfall, here are the basics if somebody could help, thanks

Loan repayments: Interest Only $ 2050 per month / $ approx $ 24500 p.a
Rental Income: $ 250 p/w / $13,000 p.a or $ 1010 per month after agent fees deducted.

Tax rate 30% (60K p.a income)

Thanks.