How to estimate Preliminary Tax on Rental Income?

My parents bought a new house in July 05 and rented out old house from that date - my dad handed me a bunch of receipts the other day and asked me to work out his tax liablility - have worked out his liability for 2005 - the problem I have is working out his Prelim Tax for 2006 - I have a figure up to Sept 06 do I just pay a percentage of this or how does it work??

You can pay 100% of the previous year's tax or 90% of the tax due but if you underpay then you are open to penalties.