The arc marker is an ideal tool to mark the 180° front arc (90° marking is also possible) on round lip bases. We have three versions: Acrylic glass black and red or MDF.
http://www.tabletop-art.de/Arc-Marker-Red
http://www.tabletop-art.de/Arc-Marker-Black
http://www.tabletop-art.de/Arc-Marker-MDF-EN

Simply push the base into the slot, hold on to the base and the marker, and draw a line along the clip. These clips have the same curve as the edge of the base which makes marking the front arc easier and precise. You can make the marks using permanent marker, a brush or, like in the example, a pencil (which is removable).

You can glue the clips into the marker or you just plug them together. So you can remove the clips from one side in case you are using a base that overlaps.

If you want to glue them together, we would recommend wood glue, so that in case you’re using acrylic glass you have still the option of removing the clips.