Then the instructions at CHDK firmware usage/AllBest - CHDK Wiki
seem to make no sense, or I'm an idiot. (which is entirely possible
Example: If you wish to use a shutter speed of 1/50,000th of a second shutter speed, you select an Override Shutter Speed Value of 2, and a Value Factor of 1/100K. (2/100,000 = 1/50,000)
So, according to these instructions, I have to calculate the speed I want to use by setting Override Shutter Speed to something other than 1/50,000. Namely, I have to set Override Shutter Speed to "2" and Value Factor to "1/100k." And I can only set the Value Factor to something other than OFF or 1, if I set enum type to Factor.