Dan Erlewine developed this remarkably handy "look-before-you-leap" gauge for quickly comparing the wire to the slot. Simply hold the flat edge against the fret tang, note the height of the tang against the reference lines, fit the appropriate curved edge into the slot, and compare the lines again.
The gauge is machined from .018" (.46mm) stainless steel for durability in the repair shop, and it fits into any fret slot. It's a big help for refret jobs (it fits bound fingerboards), for setting the depth stop on our adjustable fret slotting saw, and for setting the cutting height of our fret slotting table saw blade. The non-glare etched surface is easy to read, and the curved edges fit all popular radiused and flat fretboards.