Cutting a 34" bass board at the 3rd fret will give you roughly 28 1/2" scale if I remember correctly and give you 21 frets with a pre-slotted board.

alot of scales used as a standard are based off a 34 inch bass scale. you can also get a 25.5 fender scale out of it using the 5th fret as the nut......its why im building this with 29 blades.....i can do a 34 scale with 29 frets, a 28.6 baritone with up to 26 and a fender scale with 24.........and everyone said "why do that, all that work and youre stuck with one scale"....... i can even get smaller scales in between that ive used for a mini guitar or bass (travel size)