Azimuth setting using channel decibel level on a mono LP?

Well, there is validity to the method assuming your anti skate was adjusted perfectly and the output of both channels of the cartridge were exactly the same. 1 db is not that much. The best cartridges match channels to about 0.5 dB. I do not care so much about output levels. I care about record wear. The stylus has to be dead upright in the groove.
A careful eye, a mirror, a bright light and perhaps some loops if you have old eyes like mine are the best way to adjust azimuth. If you have a good cartridge everything else will fall in line. If not than at least you won't be excessively damaging your records.

I asked the same question in another forum and got the answer that channel balance and azimuth are not the same thing because a perfectly centered stylus at its exact azimuth can still have some voltage differences in the two channels, causing perhaps a 1 dB difference. My cartridge specs allow for a 1.5 dB difference. So, what I unwittingly did was balance the channel outputs, but set the azimuth wrong. Using a mirror, I re-set the azimuth and found that the channel balance was off by .5 dB. I'm just leaving it alone for now. It was news to me that channel balance and azimuth are different. At least I learned something and feel less annoyed when I digitize a mono LP and see that the channel volumes don't agree.

To set the azimuth so the stylus sits perfectly "square" to the groove walls is only one "right" way to set azimuth, and it assumes that the transducing end of the cartridge (which we will call the "coils) are also perfectly oriented with respect to the action of the cantilever when it responds to stylus deflections.   The other "right" way to set azimuth is to measure electrically the crosstalk, L channel into R and then R channel into L (or in reverse order), and then set the azimuth for minimal crosstalk in both directions, which is always a compromise, because there is no one azimuth setting that is going to be best for both channels, if the cartridge is imperfectly made.  In so doing, for any cartridge that is imperfectly manufactured such that the coils are not aligned with the cantilever, the azimuth will end up at some angle different from 90 degrees.  Mijostyn and many others feel that this can result in aberrant wear on the stylus and even other forms of distortion, because the contact patches (one on each side of the stylus tip, for R and L channels respectively) between the stylus and the groove walls may become aberrant.  To set azimuth electrically, you need equipment like a Fozgometer or at least a very good oscilloscope, and a proper test LP.  Lately, I am in the same camp with Mijostyn, but I've done it both ways.
What you did with a mono LP is not quite the way to do it, as you seem to realize.  Another point that I hope is inherent to what I wrote: Setting Azimuth is never about channel balance per se; it is about crosstalk.  Channel balance actually changes very little even with very extreme differences in azimuth. (I've actually done the experiment to prove it to myself.)

This is what a balance control is for:)