Great – thanks all for your thoughts. I played around a little with the varying radii of the annulus and it actually seemed to be reasonably stable in terms of calculated magnitudes. I’ll tabulate the results and pop them here in when I get chance to sit down and do it.
Interestingly, the figures I see in running this initially are a good half magnitude lower than other estimates I’ve seen – clearly this will be due to subtraction of flux from M61 – and it will disagree with visual estimates that have no way of doing that!