For use with a box cutter/utility knife.
1) Insert the tube in the hole. Make sure that the tube is held firmly in place and that it touches the end of the jig.
2) Insert the blade into one of the slots. The blade should cut slightly into the PTFE tube.
3) Spin the tube to shave off material until no material is left to shave.
Gives a perfect 45° angle.
60° version here : https://www.thingiverse.com/thing:3479519
If you need something to cut a 60° bevel INSIDE the other end of the tube, try this : https://www.thingiverse.com/thing:3428779