1. Calculate the proper exposure for the film you are using at the f-stop you plan to shoot with out regard to filters. Use your standard metering method and pretend that you are going to make a standard exposure. e.g. 1 second @ f64.
2. Add 8 stops to the above exposure. You will leave the f-stop the same, so just double the time 8 times. 1 doubles to 2, 4, 8, 16, 32, 64, 128, 256 seconds. You can count on your fingers starting with the move from 1 second to 2 seconds, until you reach 8 increases.
3. Make adjustment for reciprocity. You should be able to find a chart for your film, or do testing to determine the reciprocity change for the film. Most of the time, the change will be given in stops based on the lenght of time calculated above. So, for a 256 second exposure, the reciprocity factor, for example, may be two stops. So you would add two stops, by doubling your last amount, twice. 256 is basically 4 minutes, so double 4 minutes twice. 4 doubles to 8, 16. Your basic exposure would be 16 minutes. I would probably go 17 minutes to compensate for rounding down from 256 seconds to 4 minutes. Make your exposure.
4. Calculate development of your film. As the reciprocity factor goes up, so does the contrast of the film. To compensate, you will need to reduce development. I keep a chart of development reduction based on increased exposure. A good place to start for your film would be at n-1 or n-1/2. Adjust up or down for later negs as needed.
Hope that helps,
Allen
