As for figuring out proper exposure when shooting multiples, there were a couple of sound approaches.
If you are layering a regularly toned scene over top another (especially if the camera is tripod mounted and you're photographing the same thing, except the stuff that moves between frames) you have to reduce each exposure the right amount so you don't overexpose things.
It's easy enough to figure out that two exposures should each be one stop under - but how do you figure out five multi exposures, or seven, or nine? Well, one simple way for film photographers was to boost the ISO setting manually. If you are using ISO 100 film, then for five exposures, override the ISO to 500 (one click past 400). For seven exposures ISO 700 would be ideal (so ISO 640 is closest).
The trick is to remember to set it back properly when you're done. Cameras without DX override need not apply - but then they usually didn't have a multi-exposure mode either.
Now, if your multi exposures are made up of separate parts layered onto a darkish background, then don't make any adjustment to the exposure or ISO. For instance, adding a long tele shot of the moon onto a regular twilight/night sky scene doesn't require you to underexpose the regular scene or the moon - they just add to each other. I know, a cheesy example - but I did such stuff when I was younger.