An MK10 hotend requires a PTFE tube that is 34mm long, with square cut ends.
This guide makes it easy to repeatably cut your PTFE tube to 34mm. The distance from the knife slot to the end is 34mm, making it easy. It's pre-configured for an MK10, but you can use the Customizer to change the length or PTFE Tube diameter. I've included the OpenSCAD source as well. Enjoy!
This should be a pretty easy print for most.