for a four banger you need 19mm center to center at the bridge. you need about 9 or 10mm center to center at the nut. You can start by drawing a 34" line on a long piece of paper and scribing out 28.5mm from each side of the line at the bridge, then scribe about 15mm from each side of that line at the nut. see what looks good for you. at that point you have the spacing for the strings. I add 5 mm from the center of the E string to the edge of the fretboard and 4 mm from the center of the G string (damn I love that one) to the edge of the board.
As mentioned, Stewmac offers a nice calculator, just punch in the number of frets, the 34inchs and voila...
There's always the search fucntion at the top of this page...
and most important, if you wanna build a bass, I bet you have one lying around your place. Go buy a 15$ Caliper and take some measurements, its the fool proof way to go. Youll need that caliper for a lot of steps in building anyway.
Good Luck
-Mario-
