scale length is from nut to bridge.
to be REALLY fussy it's nut to saddle on the thinnest string minus the thickness of that string.
but given the resoultion of projectors that schools use (we're not HD!!) just the raw length will do.
if you're doing a martin D18 then its 25.4" or 645mm
either go to you fav supplier and buy a preslotted fretboard of that scale or go to
here and it'll calculate the fret spacing for you to cut them yourself.