When arbing horses BF's reduction factor is due to a non runner, so in most cases the bookie will apply a Rule 4 deduction. In all the time I've been MB'ing I've only had 1 rule 4 which cost me some profit, every other one has always made me more or no effect if your horse doesn't win.
If you are using SNR's then non runners can cause problems due to the high odds. A NR with a 50% reduction factor would turn your 11/11 into 6/6, meaning you will have laid too much.
Which bookies / which sports, comes with experience. It's unlikely anyone will divulge their own golden goose and once you've been at it a couple of years it comes down to who hasn't gubbed you
"All you need to ask yourself is do you feel lucky? Well do you......Punk!"