For my breeder racks I used a ten inch spacer following the guidelines in the DIY article. Be warned, this is pretty tight and I found it difficult to toss a lot of food at a time on to the food side so it takes a few extra minutes on days you are stocking up on your feed pellets.

On my birthing rack, I left 8 inches of space (+6 for the tub = 14 inches) and limited it to five levels of two tubs and that works pretty well for maneuvering hands and scoops of feed.