FretGuru Luthier Tool 

Can you say versatile? This 8-in-1 guitar rocker luthier tool will measure string gauges (in ten thousandths of an inch!), as well as string action. If that weren’t enough, it can also measure fret height, and nut slot height! And it’s a fret rocker, too!

If you are building your own instruments, this is a must, and if you are doing your own setups, it’s a super time saver!
Click the image or here for the amazon link!