Hi Mike,
regarding YZ CMi coordinates, please see my post here.
These red dwarfs are very high proper motion objects, so they will move a lot and you need to double check where they are in the sky now.
YZ CMi typically has V= 11.2 and B= 12.8.
I downloaded your image and, using the 125 comp in the AAVSO chart (B= 13.517) I got B= 12.68, not 15.41.
You are probably not including the star in the photometric aperture due to the coordinate mismatch.
Cheers,
Sebastian