Hello David,
I have two tips for you:
1. the exposure time of the prominences is determined by their brightness. If this is too low in your setup, you will need two separate exposures for the surface and the prominence.
If the half-width of your filter system is very narrow, you can usually manage with one exposure time for the surface and prominence. This should not be the case with your setup.
2. I always make flats in Firecapture (FC), i.e. the flat is already taken into account during the exposure. This is very easy in FC and the stacking program has no problems with dirt on the sensor because the images are already clean.
I create the flat with a triple layer of transparent film in the light path or, in your case, in front of the optics with a brightness of around 60 - 70% of the histogram.
cs Harald
www.unigraph.de