I think they have 6/32" of tread depth/rubber, and shaving to 4/32" leaves only the 2 circumferential grooves, so the tread is only 2/32" deep, or 1/3 of the useable rubber, if you run all the way down to 0. With even wear, you should be able to run it all the way down until it cords. So less than 1/3 used up so far, so you may be able to extrapolate from that.
My first set of NT-01 lasted a really long time. I went to other tires, and my second set lasted less time, because i was faster. But i still got decent life out of the 2nd set, i can look it up and see. Just not "forever" like my previous set from 1-2 years before when i was slower
but still a decent amount.