I was taught 30 years ago, by Kim Walker at Guild, that proper steel string nut spacing is
equal distance "between" the strings. But over the last few years
I've seen more precut and factory nuts cut center on center
which makes for very odd spacing.

Fender American Standard nut

Almost .020" difference!

GraphTech pre cut nut.

Almost .030" difference!

It seems like the rules have been thrown out the window
but I think it's just easier to program the CNCs to cut center on center and most people don't seem to notice.
But I do.

At Guild we used pre cut nuts that were pretty accurate as are the Taylor nuts and some others now.
If we had to cut from scratch Guild had templates that were pretty accurate for one string gauge.
But when I went on my own I found templates and rulers not to be good enough.
With different nut widths and all the hybrid string gauges I started using a formula
I found in a Stewart Macdonald catalog back in the late 90s. I now use it on every
nut I cut from scratch. It's accurate with any width and string gauge.
Here's a quick tutorial:

After the blank has been properly fit cut the 2 outer string and file to the correct width.

Measure the outside distance. This is "X", or 1.442" for this nut.

Then subtract the total width of all the strings. This is W, or .148" for a standard set of .010 - .046.
Then divide by 5 and you get the proper string spacing.
X 1.442" - W .148"= 1.294" / 5 = .2588
Set your calipers to this spacing and use it as a guide to space the strings.

Different spacing on this Strat

Now this is proper spacing and will work with any gauge or width!