I created a simple tool to help you find the center of a cylinder. Included here are one SolidWorks file with various configurations for the small (25mm), medium (50mm), large (75mm) and XL (100mm) versions; this dimension pertains to the overall width or length of the device. Also included are the STL files.
Note that the diagonal was created with a slight (0.75mm) offset for the width of the marker (see pics). After some trial-and-error I determined that this was a pretty good offset distance using an extra fine point Sharpie to draw the line, although 0.5mm could work as well if you really tilted the tip towards the diagonal. It really just depends on the width of the pen or pencil you're using.

This is similar to the project below although I created my own solid model file with various configurations for the sizes and added a chamfer to the bottom to help you draw the line as well as the offset mentioned above:
https://www.thingiverse.com/thing:1605373

Doesn't Matter

No

I'm using a Monoprice Select Mini (for some reason Monoprice isn't an option in the printer brand). The 45 degree chamfer on the bottom printed just fine without supports.