I was practicing less and less because I was tired of spending so much time making my guidelines using a ruler so I decided to design and 3D print a dedicated tool.

You can access the design on OnShape using this link:
https://cad.onshape.com/documents/db46d5ae4649cfcf55ed6899/w/55125696aa2237cb1d93bdf7/e/65546bfc1a4d5d8e62350b58
Feel free to edit it to adapt the size.

I made my tool with 2 sizes 0.5cm and 1.5cm (for 1mm and 3mm broad nibs)

Between the 0.5mm holes there is 0.5mm so you have also 1.5mm between the top 0.5 and the bottom of the other 0.5mm. In addition, the space between the 1.5mm hole and the top 0.5mm is also 1.5mm.