Mate welcome to the fustrating world of astro-photography ( film).
I would say 90% has been covered her by the group,
but my 2 cents worth.
firstly make up a focus mask or Hartman Mask.
http://rao.150m.com/Focusaid.html
( nice big piece of black card over the front of the camera, with three large holes in a triangle on it) this will allow you to focus much easer on prime focus or via an eye-piece adaptor. if out of focus, the comet will appear as three seperate images, as you approach focus, it will converge to a single image. Then take mask off and shoot.
As for bracketing shots, I shoot a few exposures either side everytime I do photo work.
ie on Saturday I shot with 400 ASA film on a 80mm lens. exposures where from 1/8, 1/4, 1/2, 1 second, 2 seconds, 4 seconds, 8 seconds and a bulb of 10 seconds. I then refocused and repeated.
From experience shooting the 2002 exclipse, where I shot three rolls of 36 exposures each ( swaping camera half way through the totality!!!) those 90 plus images resulted in about 10~12 wow images and two that finaly got published. Thats the name of the game with film. Digital is no better, with some nights I shoot 50 images and end up with a hand full I'm happy with.
Only those close to God and with no social life further than a meter from their scopes...
can shoot five exposures and get five images that are drop dead stuff...
cheers and just enjoy it
Tony
ps I use and old richo manual SLR, with no batteries in it, because even the LED for the light meter in the view screen can bleed over on to the film.
Dark is good, keep it simple and experiment...