This is what my previous shape actually produces.
The circle only touches at four points. The correct shape is obtained by calculating where the semicircles meet the circle.
The centres of the ends of the filament lie on a circle with a radius of the hole plus half the filament height. The end of the filament is then offset inwards horizontally by half the filament height. I.e. the slicer samples the layer at the central tip but the filament touches the circle on a tangent.
To make the shape geometrically I make a teardrop with a radius of the hole plus half the filament width, split it in half and shift the two halves together by half the filament width. As can be seen here that goes through all the filament tips, i.e., where the slicer samples.
This is what it looks like relative to the target hole.
Interestingly it is the same as my previous attempt at the top, bottom and sides, i.e. the only four points it touched before.
I made a test with the new formula.
The plug gauges all fit, but more snugly than before, so this is definitely a better solution for supporting a bearing in a pocket. At every layer it should have a tangential support from the rounded edge of an extrusion.
Here is a close up of the 6mm hole that is aligned on a layer boundary and the 1mm hole above it.
I have updated NopSCADlib on Github to use this method.