Here is what you need to do. Measure from the center of your highest string to the center of your lowest string. Lets say it is 3.75 inches on a 6 string. You will need to divide this number by the number of spaces between each string. In this case there are 5 spaces. So if the string spread is 3.75, we divide that by 5 and we end up with a string to string spacing of .750.