28.4mm is the minimum thickness to install new pads....
27.6 is the minimum thickness when the rotor must be replaced...
"Installation of new brake pads is approved only provided that the brake disc thickness exceeds the specified dimension (see table).
Brake discs must be replaced when they reach the nominal thickness (see table) minus 2.4 mm!"
Nominal size (diameter x thickness) mm 340x30
Minimum brake disc thickness stamped on brake disc chamber mm 28,4
Yes you need to measure at 3 spots and even even on a few different position on the disc...
I got this for $25
https://www.harborfreight.com/6-in-l...per-63714.html