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hippy
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 Crossride
25+((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)=26
Because 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 🙂