Andy, the basic "rule of thumb" Alan mentions, and the one you may wish to remember, is: Expose for the low values, Develop for the high values. This is the basic concept of the zone system.(posted 9412 days ago)Exposure/density in the low values/zones is determined by the chosen film EI. Choose a lower EI and you have increased exposure/density in the low values/zones; choose a higher EI and you have decreased exposure/density in the low values/zones. Density in the low values/zones is determined more by initial exposure than by subsequent film development.
Density in the high values/zones is determined more by development times/temperature/dilution than by the EI chosen. Develop for longer times/at higher temps/in stronger dilutions and you increase density in the higher values/zones at....and this is key!....a greater rate than in the lower values. Develop less, and the opposite is the case.
Sergio.