It's not possible to make a guitar that plays in true pitch in every key. The notes you get are always an average - but that goes for almost every instrument except brass. Two points - (1) old Spanish guitar makers sometimes made the compensation by cutting a little (about 1/10") from the top of the fingerboard; (2) if you're trying to adjust a nut and overcut the slot, it's a lot easier to fill the slot up with epoxy adhesive and file back down than it is to try and fit a new nut. And if you're trying to change the nut on a Martin, saw it off with a fine X-Acto saw or a veneer saw, pare the mess out with a chisel and fit a new oversize nut. The glue they use is so strong that if you just try to chip the nut off (which works on most other makes) you'll spend the following day trying to glue the top of the fingerboard back together.