Tile URL-Function

The TK25 grid overlays the OSM and the gpx file with the boundary.
The state of Brandenburg is covered with 298 squares (MTB).
These quadrants within the bounding box coordinates.
The first quadrant is at the top left and the last quadrant is at the bottom right.
Actually, it is synonymous as a generated grid as your tile function only with more and smaller quadrants.

If you want this smaller quadrants than exists tiles - you need to create your own map type. I described it.
You create map tiles with your lines by special programs. Maptiler and so on. After generating you got folder with zoom subfolders with tiles and test html file to check what you got.
You upload this folder to your site to accessible place and create map type as overlay. And enter GetTileURL function like you got in file but replace local place to your site folder.
In this case will get default map tiles and over it your grid tiles.