Set posts at a fixed spacing and the last bay ends up a stubby half-width that looks wrong for the life of the fence. This works backwards instead: it finds the bay size closest to your target that divides the run exactly.
Notes
Why even bays matter
A run of 2.4 m bays finishing with a 0.7 m offcut is the first thing anyone notices. Dividing the run evenly costs nothing and the difference in bay size is usually under 100 mm.
How deep should posts go?
A common rule of thumb is a quarter to a third of the above-ground height, and at least below the local frost line. Ground conditions, exposure and fence height all change this.
Picket spacing
The gap is set by taste and privacy, but pickets and gaps have to divide the bay evenly or the last one is a rip. The tool adjusts the gap slightly to make it come out.
What this does not do
It does not check whether the fence is within a boundary, needs consent, or can take the wind load at that height. All three catch people out.