What's helped me (thought it might be 'cheating'), is to go through my old digital photos before I know I'm going to be using my Leica in a dark environment. I try to find pictures taken at where I'm going, or in a lighting situation similar and use that as a starting point. For example, there's a bar some friends of mine own that I go to often, and I have some pictures taken with my DSLR from when Sean Peyton (head coach of the Saints) showed up to our pub quiz. A lot of the pictures were 1/15 at 2.8 (as fast as my zoom as gets) at iso 1600. I then go through the pictures and get an idea of the range of lighting and exposures and essentially go back with it in my head that at iso 400, and at 1.5, I'll need to shoot at between 1/8th and 1/30th depending on where I'm pointing the camera and what lights I'm looking at.
It also helps that I remember some of the exposures from when I carried my t90 (has a spot meter) to shows with me. So I know if I go to, say, One Eyed Jacks to see a band, that the last time I shot with my 35mm lens, 1/15th or 1/20th, at 2.8, iso 3200 and I got good shots. So, when going to One Eyed Jacks and bringing my Leica, I know I need to shoot at 800 and bring my Summarit.
Tomorrow I'm picking up an old Gossen Luna Pro (I guess they're ok?) for $30 that is supposed to be dead accurate. This should help out with the low light shots, I hope.
Another thing (which has been mentioned) is that negative film has a pretty wide exposure latitude. Sometimes I'll just set the camera to 1/15th, f1.5, and iso 400 (or 800 depending on how dark it is) and shoot away, occasionally lowering the shutter speed or stopping down a stop when there are the occasional bright lights. Since I use a Summarit as my low light lens this works pretty well, since if there happens to be a bright light near my shot I usually try to avoid it as the Summarit will flare like hell because of it.
Anyways, wide open, slowest speed you can handle, and highest iso you're comfortable with (800 should suffice) is what I go with when inside bars/venues.