Hello again Dominic,
>My reaction would be to query what the point of publishing these absolute magnitude values is, if the numbers produce such wild predictions.<
I think you might be assuming that the MPC/IAU/CBAT have a team of dedicated comet experts calculating the absolute magnitudes, but they don’t. They are a small group of people who are really, in the case of the MPC, orbit experts, but not light curve experts. When something goes wrong with their computers they take months to fix because they have little spare capacity…this is currently the case with their orbital elements for software packages data….not updated for months!
Really, where magnitude data is concerned, it’s the amateurs who know the most about likely comet performances, based on the developing light curve over recent months and previous apparitions. The guys at the MPC have to put a value for H into the data and presumably it is generated from the initial discovery mag, taking 5 log delta and 10 log r into account, but with little feedback from observers, apart from the magnitude column in astrometric measurements. While the astrometry will only be accepted if it is of a high standard the magnitude data is often very inaccurate. In contrast, the magnitude data found at COBS ( https://cobs.si/ ) is likely to be far more accurate and would be far better for deriving a comet’s likely performance. If the MPC analysed data from, e.g., the BAA’s Comphot software I’m sure their predictions would be far more accurate, but they just don’t have the manpower to keep up to date with all the brightening comets around at any one time. Even if they did, comets are always going to be a law unto themselves as they approach the inner solar system!