I now start at ISO1600, 1/320 f4. Seems to work in lots of venues I shoot. Sometimes I do have to go to 1/250s.
Get there while the lighting is being setup. The lighting, center spot will be adjusted and aimed with a roadie usually standing there, adjusting a mike stand. Spot meter off his face and actually take a shot. Check for overexposure of the face. The rest of the image doesn't matter, imo. Change the settings, if needed and shoot again. If no roadie, find something on the stage in the main lighting and meter off that. Find something neutral in color. There is always something. Do this with the camera in Manual.
Once you think you have it right, wait till the show starts. Get as many shots as you can in to get a comfort level before it actually does start. Once you actually are shooting the show, check the first few images for exposure. Histogram and blinkies. Reading a Histogram requires some experience, imo.
I would also adjust the LCD brightness before hand to more closely mimic what the real image will look like when importrd into PP software. You can do this at home. Having the LCD too bright or too dark may make you doubt the Histogram.
I pretty much don't do much preparation anymore other than the spot meter routine, a couple of chimps I am usually good to go. I will review some shots, when I get a chance, during a show and make some minor adjustments if I need to but I prettyy much get it right from the get go.
Good luck and let us know how it goes.
PS: Don't go chasing the perfect exposure after every shot. It's a waste of time since the lighting will be constantly changing.