SixStringers Nut Slot Gauge: ends the agony of flimsy Feeler Gauges and the nightmare of trying to get them to lay flat and steady on point on a radius fret to read against a bouncy string.

Accurate nut slots are critical to how the guitar plays and sounds. Sloppy nut slots cannot be compensate for no matter how expensive the guitar electronics and hardware are. The play and sound of every guitar depends on True Level Frets, precision nuts slots, and precision string action, get these three right and you rarely need neck relief, and if needed it will be tiny so as not to screw up your action and string response, intonation, bends, and sustain.

SixStringers Caliper Adapter: Fits on the low jaw of the calipers, has a slot for the fret tang that allows precision measuring the mushroom head of the fret. Frets are not precision made which is why standard practice is to level new frets after install... really stupid.
How about using Commonsense: measure frets before install, cherry pick frets that measure the same along the length of each fret and the same for every fret. You need two packets of frets, an additional $15, but you save having to level frets after install - hours of laborious time saved and also not leveling frets means you get the full life of the frets Vs. grinding off fret life.