Thank you very much.
You are right about the diameter, it is about 75mm or 3" ...but for spacing I usually use about 20 cm just to allow for versatility of the plants i put in the system.
It really all depends on which plants you going to plant and how big you want them to be...
But 20 cm should be good for most of them...
So draw a dot every 20-25 cm, and then drill a 75mm hole.
Will be great to see your project after you done.