Hi! I can't recommend exact measurements for you, but i've recently been planning out the same thing, and i've found that the buttons just need to be at a 'comfortable' length from each other. If your fingers can reach them all without an stress and you hardly notice the change from button to button then I think you'll be a-okay with your setup. You don't even have to put all your buttons right next to each other in a straight line, you can create two "main buttons" and then some above or below. Anyway though I don't think you should concern yourself with measurements but rather how it feels to play with the buttons from where they are. Try setting them up at different lengths apart and in different configurations and see how it plays out before you finalize a decision.