Walter-
There's no magic to this stuff.
The nut, 12th fret, and pin width measurements are not enough to get you where you want to be, unless you know the amount the bass and treble E strings are inset from the edge of the fingerboard.
The nut width and 12th fret width are fingerboard measurements; the pin width is the string spacing at the bridge. You have to know the string spacing at the nut and 12th to complete the picture.
If you get out a sheet of posterboard and a straightedge and a ruler, you can figure this out fairly quickly with a full-size drawing, for your particular guitar project.
You can also do the whole thing with arithmetic, if you are not a 'visual aid kinda person'.
Draw out your fingerboard taper from nut to 12th.
Decide on your string inset and string spacing at the nut. Inset is usually in the 3mm range on my steel-string guitars- more on the treble side, less on bass. If you bevel or round back the fret ends a lot, or use light strings or short scale, you may want to increase the inset- check the measurement on a guitar you like to play. Generally the inset is constant or increases as you move up the fingerboard. Having the strings slip off the edge of the fingerboard makes a guitar practically unplayable, so I tend to err on the generous side with inset.
This will give you the string spacing at the nut and the 12th fret-just extend the measurements (drawing or arithmetic) to the bridge and you will have your pin spacing data.
BTW, this is an example of why it's a bad idea to make up many bridges (in a batch) unless you've decided on a 'standard guitar model', since the pin spacing should vary as the fingerboard and scale change. It's just one of many areas where it's better for us hobbyists not to emulate some of the pros too closely.
Cheers
John