Get accurate string spacing without the use of complicated tools with this easy-to-use String Spacing Ruler.
"Mark outer string locations with pencil lines (high and low E strings on guitars), usually about 1/8" in from fingerboard edges. Turn ruler over (use marked side). With wide spacing toward bass side, narrow spacing toward treble side, find group of marks (six for guitar) which best fit your two pencil lines. Use a razor knife or thin mechanical pencil to mark the string positions through the ruler's slots. Choose group of marks from the same set (closer or further from ruler's edge). If you mix marks from the two sets, strings will be irregularly spaced."