I've always just used the trusty old eyeball.
1 tip is to run your TOP half a little over your bottom. (top going around and about a 1/4" over the bottom.)
Something I like to do is a trim strip of a 3rd color overlapping both on the leading edge. (adds a detail to it that I just like and is pretty simple to do.)
It's a great way to use that chrome or gold monokote that really catches the eye.
Bottom is white and my god I have alot of clover in my yard!