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what do you mean by rim width? outer, inner or bead seat?
If you meant inner, I agree but would maybe stretch it to 60% - 65%
Jaap-
inner width, like 18mm inner, so a tyres 50% of that size would be a 36mm one (thus 32-38mm).
Personally, I agree with your measurment (60-65%), I did ride on 42mm with 17mm internal and work out surprisingly well because the tyres is already a good balloon shape.
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Tester mentioned a month or so ago that the ratio of rim inner width to actual tire width should be between 0.5 to 0.75, at least for slicks.
When I asked Syntace about tire options for their 30mm rims (24.5mm internal width) they said 33mm was perfectly safe, and pointed out that using a relatively wider rim means more air volume in the tire, which increases stability.
Most of the new aero rims from zipp, Hed, enve etc. seem to be intended for use with tires that match the external width of the rims, so that air flow isn't interrupted at it passes from tire to rim or vice versa. Not so relevant for functional bikes but that seems to be the way things are headed.
edscoble
Rudder2
Depends on tyre. Been thinking a lot about this recently with the popularity of + tyres. I wouldn't fit a tyre to a rim that was more than 45-55% of the tyre's width. Particularly for long distance.