SPACING is an interesting topic and in my opinion, type and species of plant IS a (if not the) most important consideration.
When looking for specific discussion topics, I find it easy to use the little search feature near the upper right hand corner of all screens on this site. Just type "Hole spacing in rafts" for instance into the little white window and click enter or on the little magnifying glass image to its right. You will see a large number of discussions in progress. Here's a link to one
I hope this helps.