I was looking through the specification of the AAVSO extended file format at:
https://www.aavso.org/aavso-extended-file-format
It says the KMAG should be the instrumental magnitude of the check star, unless it is an “ensemble” observation in which case it should be the calculated magnitude.
However, I’m confused by example #2 which is not ensemble but they seem to show the calculated mag of the check star rather than instrumental mag. The KNAME is 105 and all the KMags are close to 10.5.
If this a valid entry? I would assume all instrumental mags would be negative since it is defined as:
mag = -2.5 * log(flux)
It would take a very small flux (< 10) to get a positive instrumental mag.