Calculating bottom bracket spindle width

I’ve got an old set of square taper FSA tempo cranks (2x) that I want to swap from a single speed 120 OLD bike to a 1x10 135mm OLD one. Any tips on how to calculate the spindle width for correct chain line? I can’t find anything for that model on the manufacturers website, and i don’t think measuring the one on the single speed will work with the different chain line.