What makes board-on-board different
The fetched Omni page explains that a board-on-board fence solves future wood shrinkage by overlapping adjacent pickets instead of leaving a fixed gap. That overlap preserves privacy even if the boards dry out and narrow over time.
Formula used for the picket count
Number of pickets = (Fence length - Picket width) / (Picket width - Picket overlap) + 1
Omni also recommends a rule of thumb for overlap: use about 24% of the picket width, which is twice the worst-case 12% shrinkage.
Omni's practical overlap examples
The source article gives a few sample overlaps: around 0.8 in for 3.5 in pickets, about 1.3 in for 5.5 in pickets, and roughly 2.8 in for 11.5 in pickets.