John, 1/8" inset at the nut is pretty much standard, and I think that's what you'll find on most factory guitars. If you do your layout the Cumpiano way (full size, I'm tellin' ya!
), you have your nut width determined first, and you'll inset your strings from that dimension. However, before determining neck width any further, you'll have to set your string spacing at the saddle, and draw your outer string lines from nut to saddle. THEN, finally, you can determine your fingerboard width. Assuming a 20-fret board, determine the imaginary 21st fret position--that's the end of your fingerboard. Draw a horizontal line at that point, say, 3" long, just to be safe. And finally, the payoff--at your neck/body join (14th fret) mark points 3/16" out from your string lines, draw lines from the nut width, through those body join points all the way to that 21st fret line, and there's your fingerboard/neck width! The extra 1/16" at the body assures that, as the strings get looser toward the middle of their span, they won't slip off the board. That's the Cumpiano way, and as beginners we can't go wrong following that method.
Whew! I'm goin' to Disneyland!