Hi Andy,
I hate to be a wet blanket, but "Bi-hex" is completely different to the drawing Bry has, which is a dodecagon..
Regrettably no grease monkey tooling will work for caseback removal, where only the right style of tool will work. When the height of the polygon is in the order of 1/2mm ( 20 thou), your average socket-set socket lead-in renders its use pointless.
As for tolerance, 38.9 +/- nothing sounds about right, if Bry's measurements are correct. This really is a case for "spot on" sizing, if damage is to be avoided. Or as an old timer was reportedly heard to say "I don't work to tenths(of thous), I do it exactly"

Ned