As close to the center as possible in the deepest part of the tread. Take readings from multiple points around the tire and use the lowest (there should be minimal differences between the readings). Most front tires start with around 5/32 while the rear start with around 11/32 (differs by brand and model). Wear bars are generally at 2/32.
I recently purchased two Michelin Commander II tires on sale. My front tire was the one most in need at 3/32 while the rear was 5/32. Once the tech got the bike on the rack, he discovered my rear tire had two 3" long cuts and a puncture hole (didn't go all the way through, so not leaking). Glad he spotted it before I took-off on my 1,000 journey next week. Dusty now has two new shoes and rides better than new!
Be safe out there...
Some folks seek the comfort of the straight and narrow. Me? I prefer the twisted road.