Essentially put a capo on the first fret and the last fret before the curve on the neck. That takes the nut and bridge out of the picture. Slide a business card under the strings. It should just fit. If there is a lot of play, or the card won't go. Adjust for the curve.
Dreambow Semi-Hollow Body, G&L L2.5 Fretless, Ibanez BTB 776, Sadowsky NYC, Pedulla Custom Buzz, Dragonfly Rose Acoustic, Double Bass