That depends on the tub you use and where you are feeding from. You need to leave at least two inches between the tub bottom and the top of the rack under it so you can get to the sipper and the food.. It also helps with air flow and light.
when building a rack i'll use a ten in spacer to separate the tops of the tubs. with the tub in place, lets say they 5-6 inches so that leaves me 4-5 inches of space
1.1 het pied
1.1 butter
1.0 lemon pastel yellow belly
0.1 lemon pastel
and the addiction begins!
feeders(rats) - breeders(1.9) very happy male!
feeders(mice) - 2.5 breeders
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.