While there is the option to buy a nut that has pre-cut string slots, cutting the slots yourself ensures that the spacing (and depth) will be absolutely what you want.
How to determine what spacing to use is done by using math. But don’t worry, it is math you can handle.
While there are other methods to determine string spacing, this is one which we call the A – B – C ÷ D formula, as in “A minus B minus C divided by D”.
A – Total width of fingerboard at “nut width”. Nut width may be slightly skinnier than overall neck width.
B – Total width of string for what you intend to use where all string diameters added together. For example, if we took a 5-string set of La Bella Low Tension Flexible Flats model LTF-5A, the widths are .118-.100-.070-.056-.042 and this adds up to .386″ total. When this is converted to metric .386 x 25.4 (the number of millimeters in an inch), the total we get 9.8mm which is then rounded up to 10mm.
C – Desired space from edge of outside strings to edge of neck multiplied by 2. If 4mm is used, then 4 x 2 = 8mm.
D – One less than quantity of string (5 – 1 = 4) because there are 4 spaces in between the strings on a 5-string electric bass.
Using our example measurements on a 42mm nut width, the formula with values would be:
46 - 10 - 8 ÷ 4 = 7
…meaning a 7mm spacing between each string.
A hell of a lot easier to just use this ruler:
http://www.stewmac.com/Luthier_Tools/Types_of_Tools/Straightedges/String_Spacing_Rule.html
Maybe you can carry these?
in your formula you use 46mm for A but the nut width you specify is 42. Why the difference?