[Scargoes]

The thing to remember is that the outer two tread blocks become one at about 4.5mm depth. Only way to know how much is left is to check the tread depth holes that are molded into the tread. Look for some rectangular holes near the edge. That is where you take a tread depth measurement.

Edit: I have circled in red the holes you should look for. The groove identified by the green line is not as deep as the other grooves. This is so as the tire wears the outer edge becomes more like a slick with less voids and less tread squirm.