Yesterday I watched this video:
After watching it I got my caliper and measured the 3 adapters I have used. I bought one from E-bay, very cheap and I bought two here in Norway and they where cheap for Norwegian standards.
I looked up the “flange focal distance” for my Fuji X-mount and de M42 mount and I found this site: https://en.wikipedia.org/wiki/Flange_focal_distance.
For the M42 the distance from the film to the lens mount is 45.46mm and for the X-mout it is 17.7mm so the adapter has to be 45.46-17.7= 27.76mm
I measured my adapters and the one from E-bay was 27mm on both sides, 0.76mm to short. The other two are seemingly from the same (no-)brand, one of them was 27.95mm and 27.85 so a 1/10 mm difference from one side to the other. The other was exactly 27.80mm, only a Little to big.
I put the other two away, the E-bay one is the same height around but almost a millimeter to short. When the adapter is to small it might influence your infinity setting but you have some wiggle room here. When the adapter is not the same height it influences the sharpness, specially in the corners so this one has to go.
I might buy a brand one one day but i looks like it is a gamble no matter what, at least I have one that is good. Who new it was so hard to make these things.