I have a lot of these curves in different shapes. Finding the most convex points along the axes by eye is not an option, you need absolute precision.
Maybe someone can suggest a script that would select the most distant point on any axis. Or maybe there is already a built-in command in Blender to do the same thing.
If it would be easier to do the task with Mesh rather than Grease Pencil - you could convert curves to Mesh as well, no problem.