Purchasing new parts for new build.

[Clive S]

so 20mm is more than enough, great! can purchase long ones then to use more of my ballscrew.

Btw, when is needed 25, 35, 45mm ones? exist some kind of relation table?

Not sure what you mean with the above but in order to get some good advise you need to put some sort of drawing up and it also will be good practise for later when you what to start making things etc