Non-integer grid size
It is currently not possible to have a grid that is not an integer number of points. E.g., the following declaration in a stylesheet
<gridsize name="1mm" value="2.8346456693" />
is silently ignored. Indeed, in the source, we find int iGridSize. Would it be possible to change this to double to allow for different grids?
The problem is that Ipe stores coordinates in points, and there have always been problems with rounding errors when Ipe supported fractional grid sizes (in Ipe 6).
If you want mm- correct drawings, I suggest you draw them as if 3 pts equal 1 mm. Then, when you need the image, you scale it (for instance when including in Latex using \includegraphics) by the factor 1.05833333.
At some point I want to allow coordinates in Ipe files to be mm-correct, with the conversion to PDF points happening automatically when saving in PDF.