II do not know of a way to set it with out some test equipment, at least a dvm that can measure the current to the PA section.
Using either a current meter on the PA input, or an RF power meter on feed to the antenna, I would start with the supplied mic, and then do the "HEEEELOOOW" test in the wireup section on the HF Signals web page. Use that current or power level as the desired level, and then adjust the audio drive to get the same level of output, or lower. Then go and try the digital mode of choice and get feedback on signal quality.
I have not used the digital modes with the uBITX, so there maybe flaws in the above. I did use the method to verify an amplified mic setting to insure I did not overdrive the mic input, and verified as much as possible with the simple spectrum analyzers (SA) that I have (Cheap RF Explorer and a RSPlay RSP2 used as an SA). As far as I could tell there was no splatter.
Above are just my thoughts. You are still responsible for the signal quality, and there may be better options, like going to a local club that has access to a SA or other test equipment.