The 12.5 number is just an offset so that the GridSplitter is between the Maximized item and the minimized ones. If there's no offset the GridSplitter will be on top of the column with the minimized items, because the GridSplitter's column will be 300px and the MinimizedColumnWidth will also be 300px (at the moment it's 287.5). You can just try to remove this offset and see what happens.
I hope I was able to help you and if you have further questions please feel free to ask.
the Telerik team