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I was talking about it the other way, if you have a 18mm inner rim width I wouldn't mount any tyre smaller than 28/30mm (or else you're gonna run into trouble mounting them after a flat). Bigger is just fine!
Jaap-
Ah, well a lots of road wheels are actually 17mm internal and have been mounted with 23mm tyres.
18mm internal you defintely can get away with 25mm, While you can with 23mm but the risk is that the rims actually can get damaged when hit by a pothole instead of the tyres (and mounting them as you mentioned).
edscoble
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.