Zero 'beat' is a bit of a misnomer as it's really a matter of listening for nothing rather than listening for something.
To understand, let's review the similarities between hearing SSB and CW: For SSB, the radio inserts a carrier frequency at exceedingly low value & at dial frequency. This mixes with the received wideband material and comes out sounding like true radio. For CW, this weak, injected carrier causes a hetrodyne or difference frequency between the added dial carrier and the on/off switched signal you're trying to receive from the outside world.
Now, recall that if you tube for code at too low an audio freq rather than for the traditional 600 to 800 Hz, you can make it so low as not to hear it at all. You might hear a presence, maybe not. Well, you can also tune in an AM broadcast such as WWV and make it's carrier disappear just like you can make a CW signal fade to nothingness (think: Carrier Wave rather than Continuous Wave for a second).
When you do that to an AM station while using a SSB radio or with CW - that is, make the carrier imperceptable/disappear - that's finding the zero beat.
If you had a different model of radio and could swap RX sidebands, you'd have been able to count the dial increments it takes both ways before you reach a personally-subjective detection of the carrier note again. We can't do that on the ubitx during the procedure BUT we can check it that way afterwards. Depending on your hearing, it should be 2 or 3 more clicks at 10Hz resolution on the dial, past the lowest you can hear so try that. Save the setting, then re-cycle radio power. Then try listening to WWV on both sidebands one after the other and center your dial on it. If you are more than 5hz off, then try again and change that highly-subjective zero-beat point until you are satisfied with any remaining yet unavoidable, minor error.
Oh, and as a reminder - that local (radio-produced), weak injected signal that helps pick out SSB and CW is called a Beat Frequency Oscillator.... BFO, for short.
And now you know why the BFO gets set first, before dial calibration procedure gets done.