Recently I came across this article online where this guy uses a caliper and calculations to measure the optimum, equidistant string spacing using the neck/nut width, the string gauges and number of strings, the 'margin' on either side of the first and last string and then does a specific calculation to arrive at the spacing.
Its not complicated but I made a spreadsheet and have an example of what to do and how to use:
What you do is enter the neck/nut width (or the saddle!), the margin, the number of strings and their gauges. All in inches but it converts to mm.
Use a digital caliper if possible as it will read in decimal. Measure out the margins (this case .125") and make a thin mark on the nut. lay the string down and make a mark on the opposite side. Go ahead and cut the nut slot centered between those marks. Go slow and make adjustments as needed to keep the margin distance. Once cut, lay the string in and measure the String Space as shown in red above. Make marks on both sides and again cut the slot centered between those marks. Repeat.
In the example below, the red is where the 'marls' would be and the strings should be equidistant.
Here is the file StringSpaceCalc.xls
Enjoy and email me if you need