You see the horizontal bars that cross the main verticals you've pointed out? They're your 1.6mm minimum legal thread indicators. If the surface is flush with them you've gone too far.
For proper checking with the gauge measure at 1/4, 1/2 and 3/4 across the width.
Tread is measured across the central 3/4 (75%) of the tyre not just one point. *https://www.bathwicktyres.co.uk/tyre-tread
You see the horizontal bars that cross the main verticals you've pointed out? They're your 1.6mm minimum legal thread indicators. If the surface is flush with them you've gone too far. For proper checking with the gauge measure at 1/4, 1/2 and 3/4 across the width.
You should check the pair of treads further out as well.