This center guide has a few features built in:
To mark the center of rods, you need to put in the rod flush and draw two times a line. The center is where the two lines cross.
The corner of the guide features a hole for avoiding collisions with sharp workpiece corners, when marking a 45° angle from the corner of your workpiece.
The extending leg is 14 mm wide, so that you can glue on an adhesive measure tape (usually 13 mm wide), if you want to use it for marking parallel lines to the edge of your workpiece.
You can find the Onshape model here:
https://cad.onshape.com/documents/b6bd878f553d1eeb4d368b40/w/ab34c397190a3cee1d7bb9ad/e/151853f44ff2a779d15aaaf4?renderMode=0&uiState=66cb5a353e8e314da9d8cf21
I have created the model myself from scratch, but got some inspiration from the following works:
The author marked this model as their own original creation.