80% rule

[Elijah Kerns]

I believe that somewhere I saw that one should only buy a property at 80% of what's it's worth. That's some great advice, but how do I know what the actual value of a property to see what 80% would be. I think I read this in the real estate investing book written by Brandon Turner. Not 100% sure though. Any advice on if the 80% rule is actually a thing or if I'm just making stuff up. All help appreciated.