Which we haven't given you.
Shimano Deore XT FH-M785 Rear Left Lock Nut, Cone & Dust Cover - M14 - Y3TG98040
The distance between the flats is not easily found in the tech docs (at least not by me - for you).
I suppose this is mainly because most people have cone spanners which will do 13-16 and maybe 17, and try those first. Then they have calipers to check.
You don't say whether you've actually offered the spanner up to the xt M8000 left lock nut "I'm not sure".
Do let us know what the answer is, when you find it out.
yeah not reached a conclusion yet , I've looked through the tech docs and given up after not finding the detail I was looking for, I have cone spanners from my old wheels which, measured against a ruler measure 14 & 16 mm, these new ones are larger, the spanners I have don't fit, I've tried them, I don't have calipers or want to any as I won't need them again, and trying to measure with a ruler is tricky, the axle is in the wheel, add to that my eyesight and specs I can't focus with any confidence. I was thinking just maybe someone on here would have the same hub and be familiar with the cone size, if not I placed the question on a bike web site product detail section and sent an email to the place I ordered them from.... so some thing will come up ..if not i'll do it old school and go to the local bike shop, though it still could be handy knowing the size in advance...................