I just added a new function to the VMA Simple Spectrum Analyser: Automatic Bandwidth Measurement.
This function will measure the bandwidth at -3dBm, -6dBm and/or -60dBm.
For this to work properly, you need to setup the spectrum so that only the desired signal is visible. Switch to the "Measurements - Bandwidth" tab under the spectrum view and select the required bandwidth measuremtents.
There is a problem: because the ADF4350/ADF4351 will actually produce clipping on the centre of small bandwidth signals, my algorithm needs to know the minimum frequency distance it should seek. If you see the bandwidth measurement drawing the end frequency line in the centre frequency of the signal, reduce the "Min. seek distance" field.
This is still beta - feedback is welcome!