Just a comment really about how accurate/scientific tyre wear measurement is.
At my last service my Lexus dealer advised that my tyres were getting to the point where they would need to be replaced. Not urgent, but advisory.
They assessed the tyre wear as 4mm on the NSF tyres but 2mm on the OS rear with bald areas on the inner edge.
As the tyres are from new I have recently had all four replaced with Continental Premium Contact 6.
The tyre fitter, National tyres, noted on their report that all tyres were down to 4mm, thus still fully legal.
I will never take chances with tyres so I am happy that I now have four new tyres, but my point is how accurate/scientific is tyre wear measured.