Quite inspiring—I’m not designing much, but when I do, I tend to e.g. just tweak a more or less random border width till it feels right. Math is no bad idea at all, at least it can provide a better or more consistent starting point for that ‘tweaking and feeling’ process.

I also like that you’re pointing out that an outer border’s radius should be greater than an inner’s—a lot of people seem to miss out on this particular detail, making it look weird and ugly.