What I find works is place the plant, and leave about 3-4 feet of room between before the next one, go all the way down the row like that. Then step over slightly into a new row, maybe 6 inches or so, and place these offset by that 6 inches but lined up with the spaces of the first row, and repeat. Very few plants will need more room (if so just space a bit more) and it provides a nice looking pattern. (the rows end up looking like a diamond pattern)