Home becomes a rental property

Hi,
We bought a townhouse in January 2010 and the townhouse was our main residence until we moved into a new house on 1 June 2014. On 20 June 2014 we began renting out the old townhouse. We are planning to sell our old townhouse this year (2019) and wanted to know what is the best way to obtain its market value from the time it become an investment property. As per the ATO website, my understanding is to work out the capital gain or loss you need to know its market value at the time it was first used it to produce income.

Thanks in advance,

Gabmar