I don't know if there is a "best practice." If you replace prematurely, you're wasting money and resources. If you replace too late, you're wearing out your cassette and possibly pedaling harder than you need to. The trick is to find a decent balance between the two. I like to err on the side of wastefulness. But I don't usually use a checker, so I can't say at what metric I replace a chain. Take a guess. You don't have much choice.