Ok that seems like a good way to start!
Some more practical things that could be useful to know: Always make at least one prototype and have different people test it (ideally for a bigger batch of modules you should make 5 or 10 protos), or you might end up with something like the Verbos Pan&Scan, which got produced with a big bug in the circuitry (and that’s just one example… there’s way too many examples like that in the eurorack world).
Try to stick to the standards. There’s already enough people making their own version of eurorack. People will thank you for that.
Doefer has a great page with all the sizes for panels http://www.doepfer.de/a100_man/a100m_e.htm and these have worked great for me so far.
There’s a lot of small details that might spawn unnecessary service requests, some silly examples: I got one module that had the power connected on the busboard end connected the wrong way. As long as you don’t have an un-shrouded busboard you won’t notice, but with a shrouded one you’ll connect the cable upside down (possibly thinking that since you have a shrouded busboard you can’t connect the cable wrong) and possibly damage something.
People are very sensitive to wobbly pots. Sometimes these are not bad quality ones… but a nice firm turn will make everybody more happy. Be cautious with encoders, they seem to solve a lot of problems but are tricky to implement properly and often have either a high price or a very low life-cycle. Though I guess this only applies to digital modules.
Don’t try to cram too much on a panel. make sure the fingers fit between the knobs and that tweaking them will not make you accidentally move something you don’t want.
Keep in mind that the rails need some space, keep a generous clearing on top and bottom, i.e. don’t make the PCB(s) too big)