It's possible that the aperture values in the previously mentioned table are slightly off. The correct values can be retrieved the following way:
Activate the propcase debug display: Miscellaneous stuff -> Debug parameters -> Debug data display [Props], Propcase/Paramsdata page [2]
Switch the camera to Av or M mode, watch and note the number displayed after "26:" as you set the aperture with the Canon controls.
Something like:
Canon Av, value
3.5, 355
4.0, 391
...
Not sure how much influence this has on the CHDK override in the "Enhanced photo operations" menu. Despite the correct table, I get 3.56 as the lowest selectable Av value on my sx280 (the advertised minimum Av is 3.5 on this cam).