Is it possible to find a correlation that allows one to calculate the exposure time for a different-sized print? Say I print a real nice 4x5 proof print. Everything is fine, exposure, contrast, etc, but I decide to make an 8x10 of it. Is it possible to find a correlation that will work for all images or is it too chaotic with reciprocity, subjective judgement of contrast and exposure between different sizes, and other factors? My first instinct would be to calculate the difference in area between the two prints and adjust the aperture and time to have the same total illumination density but that might not actually work.