Pete, how about k = 1.25? I don't know what the basis of this is, but using your formula it seems to produce the correct answer.(posted 8157 days ago)Way, way back doing speed testing, I just used the tables that are part of the standard. Their method for ending up with only the allowed speeds (100, 125, 160, etc) was to establish exposure ranges for each allowed speed.
For the logarithmic speed, try speed = 1 + 10* log(0.8/lux_seconds) (using a base 10 log).