As Beckmann points out, your foot length may decrease with proper support. Mine measures about 29.6 cm flat on the floor, but about 28.9 cm with a footbed that does at least a little of the support job I need. I wear a Mondo 28 in UPZ or Deeluxe and most ski boots I've tried. That gives plenty of room for a liner, by the way. I can wear the boots all day buckled without even noticing.
- the mondo size of a shell is NOT the interior length, it's the manufacturer's idea of what length foot it would fit, WITH a liner, socks, etc. That's often assuming a seriously thick liner.
- Deeluxe's idea of this is: if your foot measures 27 cm UP TO 28 cm, then you "are" a Mondo 27, not a Mondo 28 and should use the M27 shell. My experience and others suggests this errs on the side of a too-large shell. So a 27 shell may be (probably is) fine for you. You'll need to actually put your foot in some shells to see; see Beckmann's guidelines, linked in his post.
- the key is to get a good (close) fit in the ankle/heel area. If the toe box is too tight it can be enlarged, but there's not much you can do to make the heel/ankle area smaller, and too much room here causes a number of problems: toe bang, heel lift and heel spurs, lack of control in bad cases.
Good luck. If you can't try them on before you buy, expect to go through several pairs before you find a good fit. Maybe you'll get lucky, but not too many do.