Hi Bill,

due to the differences in measurements you mentioned the resulting
brightness value of course is only a very raw value. But this raw values
are usable for the goal to get a feeling which object could be
observable with the own equipment. If we get differences between
measured and real value in the range of about 2-3mag this still can be
very often smaller than difference to calculated.

I can write about my own experience. I used this MPC list methode for
many astrometric observations of comets.

But I do not use observed values from the MPC list which are older than
one or two weeks because relative big changes in brightness can occur in
both directions within this time span.

Regards
Gerhard
www.dangl.at