If one had a low illumination scene in which the brightness range were 5 zones (for instance) and the lowest level were placed at a zone III at a time of exposure of one minute (using the example given). Allowing for reciprocity the adjusted time would be four minutes. The next zone above (IV) would have metered thirty seconds and received four minutes (approximately 200% overexposure after allowance for reciprocity). The placement for Zone V would have metered fifteen seconds and received four minutes (approximately 400% over exposure after allowance for reciprocity). The placement for Zone VI would have metered 7.5 seconds and received 4 minutes (approximately 1600% overexposure). The placement for Zone VII would have metered 3.75 seconds and received 4 minutes (approximately 3200% overexposure). The fifth zone would metered 1.875 seconds and recieved 4 minutes (an approximate 6857% overexposure).
All calculations above are on the basis of a Zone V rendering for all zones (obviously Zones placement VI through VIII would have received more exposure to rendered their desired negative density. But the percentage increase would have been a 100% increase above the preceding zone luminance as opposed to the precentages given.
The other way of dealing with reciprocity and the matter of contracted development is to support the low densities through preflashing the film at a uniform non image bearing Zone III luminance. The luminance range is still being contracted. However they are contracted in a different way. In the case of N minus development the high densities are contracted in order to arrive at desired low value exposure. In the case of preflasing of film the low values are supported prior to exposure to the point that reciprocity is not the factor that it is in the example given.
In other words through preflashing we are working on the support of low value exposure so that we may not need the extreme contraction that we would be required to use if only N minus development were used.