Hello Dominic,
For many years I used to try to do this for the annual Yearbook of Astronomy. It was a bit of a nightmare. The basic problem is that comets are a law unto themselves. Only the magnitudes of the best behaved well-established periodic comets can be predicted with some certainty, and that is rarely as good as +/- 1 mag! My approach with periodic comets was simply to see how they behaved last time round, see what Guide predicted for that apparition, note the error, and then see what Guide said for the next apparition!
The MPC website can give reasonable mag predictions for well-established periodic comets, now and again, but for new comets it is often very inaccurate. Brand new comets may have been discovered in outburst or may brighten rapidly in the weeks after discovery.
As for Blanpain, well, that is a rather extreme example as it was healthy in 1819, then declared ‘dead’, then rose from the dead in 2003 – 2005. So, anyone who can accurately predict its magnitude for the coming apparition would be a genius!
I really wouldn’t worry if your predictions are ‘dodgy’ Dominic as predicting the magnitudes of comets is like nailing multiple jellies to the wall….
Martin