Determining property value

[Mark Kitchens]

Hi, just finished reading "7 Years to 7 Figure Wealth" (great book) and have a question regarding determining the value of a property. I'm assuming that when Brandon recommends buying property for 80% that we should look at the "value" not the "asking price". But I'm unsure how to best determine actual value. For instance, the condominium we currently own is valued by the county at $170,000 but when we look it up on www.zillow.com, it's value is $196,000. Which would we go with in order to come up with the 80% offer?

Thanks!

Mark