This jig allows the port length to be marked precisely. A marker pen is taped to the crosspiece, and the height set by measuring from the tabletop to the pen. Rotating the port marks a circle that is parallel to the face of the port.

This method is most accurate for ports that aren't too long in relation to their diameter. It is suitable for trimming single flared ends, or for cutting to length in preparation for flaring the second end (don't forget to compensate for the loss of length due to flaring)

This example shows a 10inch port with a 40mm flare.