I've never seen any trick to doing it easier than the hard way:

Shift into the big ring
Decide whether you need to raise or lower the derailleur, and by how much
Shift into the small ring, so the cable is slack
Loosen the band clamp
Adjust the height, guesstimating the amount from what you saw before.
Try not to maladjust the rotation
Tighten the band clamp
Shift into the big ring
Repeat

the short answer to your question is you have to be in the small ring so the cable is slack, otherwise the cable will pull the whole derailleur when you loosen the band clamp.

Can I say 'band clamp' any more times?

This one time, in band clamp...