My rule of thumb is start small with the holes. Test things out. Need to change things up? Make the holes bigger. You could easily make the spray bar, cap it and test it a few times in under five minutes.
Also buy 2-3 lengths of rigid tubing so you have some practice material. The stuff is thin and light weight so it's easy to work with.
Okay, Ill take a look at your 12g journal.
I have an eheim 2213 as well, so i was gonna piggy back off of your findings