I made this to make accurately measuring a piece of filament easier. It's almost impossible to get a piece of filament straight so that you can measure the length accurately. This holder keeps the filament straight so that you can use digital calipers to get an accurate reading.
Simply feed the filament into the hole, put one edge of your calipers in the notch, and read off the length. From the bottom of the notch to the end of the piece is 95mm, so a 100mm extrusion will protrude a little out of the other end allowing you to measure it precisely.
Check that the notch is clean!
It might be that your first layer spreads a little causing your calipers to give a slightly false reading. If so, a couple of strokes with a needle file will solve that.