I've taken a lot of shots of rock groups in the past in sometimes very gloomy lighting...
This will make some proper photographers cringe, but my technique was this (for very low light)...
Stick in the fastest film you've got.
Put the camera on manual and forget metering (my cameras only ever have manual anyway).
Open the lens up as wide as it goes, because you need all the light you can get (Use fast primes!)
Set the shutter to the slowest speed you can hand hold. For a standard lens that would be 1/60s, for a 135mm tele (which I used a lot at gigs) it would be 1/125s (or 1/250 if I was being jostled by the crowd).
Take the picture.
Who cares what the exposure or film speed is? You can't get any more exposure - the lens doesn't go any bigger and any slower shutter speed you'll get camera shake - your on the limit, so that is your lot.
Or to look at it another way, if it is going to be underexposed, it is going to be underexposed, you can't do any more than you've done, so who cares how underexposed it was?
At a theatre in a play or show you might get a mixture of light and dark scenes so maybe try a reading if the light is steady enough - but at a rock gig, especially small clubs, it was usually dim or dimmer. Even at big events, constantly changing lighting and bold colours tend to fool meters anyway, so I'd rely on film latitude and luck.
The first time you develop the film you might want to do a clip test to see how much to push the film, but personally I'd just chuck everything in Microphen (if it was Black and White, I'd use the same method for colour) and see what I'd got.
Usually, I got something
I know not everyone will agree with this approach, but I think sometimes there is a place for meticulous technique (when everything is under your control and you are making a 'fine photograph') - and a time to wing it by the seat of your pants.
Steve, that is GREAT advice. Thank you for kindly showing me the forest I was missing for looking too long at one tree. :-) I'd never thought about doing things that way and I'll *definitely* try this the next time I go and just try to enjoy the experience rather than obsessing over the details.
Truthfully, I took one photography course in the 7th grade and put the cameras away until being given a digital camera many years later. Maybe that, as another poster suggested, has something to do with my difficulty in understanding ISO as relative rather than absolute?
As of yet, the only developer I've used is D76, so I will try to research Microphen and follow your advice. I'm interested in finding an economical, easy to use developer that works well for several types of B&W film so I've got a lot of reading to do on that subect (and how to do clip testing) as well.