In searching the archives, I have found this answer. But where do find "Stub Tuning"? (I am using version 858B)
There is an easier way for trimming stubs of any length. Click Stub Tuning. Enter the Target Freq (7.175MHz in this case) Enter the starting length (30 feet in our example) Enter the Target Wavelength (0.25 for a quarter wave stub) Click the Enter button. The AIM will calculate the Velocity Factor based on the Starting Length you entered. Then it will show how much cable needs to be cut off to get the desired phase shift at the target frequency. This value is displayed at the top of the graph. A summary of the parameters is also displayed at the bottom of the graph for reference. If you want to use meters for the cable dimensions, set the meters flag in the AIM config file.