Nut width is a quick and dirty estimate and so does matter as you say, but actually, the limiting factor is the usable fret length. The steepness of angle where the fret end is filed makes a difference to the available usable fret.
Different guitars have the fret ends profiled at different angles. ie. two guitars with the same nut width may well have different maximum possible string spacings.

Keith

Woody ... just bring it to work with you, and give me a call ... I'll come pick it up and measure it for you ... I should be able to get it back to you in 5 or 6 months ... You certainly wouldn't want me to rush and get it wrong!!! :D