The Hosco Groobar is a set of steel nut slotting gauges in four different thicknesses 1.07 mm, 1.27 mm, 1.42 mm and 1.65 mm (.042". 050". 056" and. 065") that allow you to quickly set the depth of the string slots.
These little gauges are very useful when filing nut slots. They stop you from filing too deeply.
Simply keep filing until you see a file mark on the GroovBar.
A bit of measurement with feeler gauges is necessary to find the height of fret one - then add 10 - 20 thou and use the appropriate thickness GroovBar.
I recently replaced the nut in my Gibson LP Standard (a scary job) and with care and accurate measurement it went very well.
I now have a Tusq XL nut with correctly cut nut slots and a perfect first fret action.