Interesting method used by Nick over at Typophile to find the relationship between regular and bold.
I decided to do some research to help, looking at a fairly arbitrary group of 12 font families* (mostly Microsoft) and worked out the ratio between lower case thin and thick downstrokes in each case and then took an average of the 12.
It turns out that the ratios (regular:bold) varied between 0.52 (not enough difference to be effective) and 0.67 (too much difference), but the average was 0.6216.
This turned out to be almost identical to ? (golden section)