Take a meter reading as normal with the lens fully stopped down. Decide which shutter speed you want to use, clearly it must be a faster shutter speed than indicated by the meter reading. To arrive at the number of exposures needed using the faster shutter speed divide the slower speed into the faster speed. For example, the meter reading is 1/4 second at f22 but you wish to use 1/125th second at f22, divide 125 by 4, the answer is 31 multiple exposures at 1/125th. As a check that the multiple exposure is correct first make single exposure using the metered reading and then make your multiple exposure. When the film is developed the two adjacent negatives should be almost identical in density.
Originally Posted by PhotoTyler
I've used this method for nearly 30 years and found it very reliable. The image of Roughting Linn on the home page is a multiple exposure.