Well you live and learn....
Here I am, stuck in an imperialistic country ( that is, non-metric) yet I can't recall ever seeing discs over here in anything but mm! And the UK was supposed to be metric too
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. Tbh I've never heard of 150mm discs either -140, 160, 180 or 203mm seem to be the common ones. Even then, those are just guides as different designs and manufacturers can easily provide small mm differences. I've put discs on that aren't the same manufacturer as the calipers, and my rule of thumb is to use washers (not sure I'd use more than a couple of mm though) to get as close to a perfect track. I also keep an extremely close eye on the bolts so I get maximum purchase but I also am not going to crack the fork housing if they are too long.
Anyway, YMMV.