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usually tire size on wide rims is: size on a 15mm rim + half the difference between rims:
a measured 23mm tire@15c on a 23mm wide rim becomes ((23-15)/2=+4mm) = 27mm...
edit: tldr: tire size changes with half the rim width difference
svenhöek-
a 23mm tire@15c on a 23mm wide rim becomes ((23-15)/2=+4mm) = 29mm
23+4=27
Using your formula:
25+((21-15)/2)=28, but really my 25C GP4000S comes up to 30mm on a 21mm internal Mavic Crossride25+((16.5-15)/2)=25.75, but really my 25C GP4000S comes up to 26.5mm on a 16.5mm internal Mavic MY2016+ Aksium
If I were trying to guess at a rule of thumb for estimating the width of a tyre on a rim bigger than the one on which it manifests its nominal width, I'd divide the difference by π/2 (about 1.6) rather than dividing by 2.
25+(2(21-15)/π)=29
25+(2(16.5-15)/π)=26Because the extra width is actually a chord rather than an arc, this formula will always come up with an answer smaller than the correct result, but it will be closer than your version 🙂
gbj_tester
You probably want a true width of at least 28mm. In practice, that means any "25" which is 25mm on a 15mm rim will be fine, as it will come up to more like 30mm on that rim.