I'm not 100% sure if I just overlooked something, but I think that there is a bug in the pixel-position-calculation thing. In the openpyxl/shared/init.py the values for DEFAULT_COLUMN_HEIGHT and DEFAULT_COLUMN_WIDTH are set with '15.' and '51.85'. However, I think that they would need to be initialised with some value some the xlsx document (to be more precise, if you unpack a newly created document look in /xl/worksheets/sheet1.xml and you will find the following row:
<sheetFormatPr baseColWidth="10" defaultRowHeight="15" x14ac:dyDescent="0.25" />
So - if I get this thing right, the DEFAULT_ROW_HEIGHT would need to be initialised with defaultRowHeight and DEFAULT_COLUMN_WIDTH with baseColWidth.
I hope I didn't get anything wrong here how it works ;)