I think you addressed it all, in great detail.
The summary is that there are 12 columns, assuming that the content on each column, including its padding fits in the 8.33% of the panel width.
I was experimenting with the GridPanel while working on the InputBox. I ended up with using a LinearPanel with a bunch of FlowPanels and setting the width of each component from code.
I calculate the widths and use px instead of % assuming that the width of the alert is 546 if self.large else 246, which could cause it to break if the css for the alert changes. For now it works well enough.