you will need to experiment with the development times. But yes, subtract the percentage from the normal time to account for reciprocity failure. Some films don't gain as much contrast as others and J&C classic for me is an unknown. I usually use Microdol-x straight for swag (scientific wild ass guess) of 10 minutes and minus 20% works out to be now an 8 minute time. I use sodium metaborate (20 mule team borax will work also) in a second tray. One tablespoon per quart of water. The film goes in to this bath for 4 minutes. The directly into the fixer. I generally rerate the film to a lower number to support the shadow end of the scale. If the film is a 200 speed film I will, in these situations, shoot it at 50 or 25 ei. At 30 seconds, there should be some shadow detail. I am of the opinion that it is virtually impossible to overexpose film when the light level is so low. So, to review, 1st presoak the film for 2 minutes. 2nd Microdol-x straight minus your normal time. 3rd into the Borax for 4 minutes 4th into the fixer. My experience with this method is for 4x5 and larger film. It is taught at a college here to 1st year darkroom people for contrast control.