Just as a quick example:
I've got big wrists, and (roughly) have a = 19.5, and b = 7, so my j = 2.79.
Meanwhile, my daughter has a = 17.25, and b = 5.75, so her j = 3.00.
Now, to compare to one of the watches above (the 45.5mm Planet Ocean, since I have one to test with) which ends up having k = 24.38 once you crunch the numbers.
For me, 2.86/24.38 = .11, so this watch is too big for me (according to the formula)
For my daughter, 3.00/24.38 = .12, so the same watch fits her significantly smaller wrists (again, according to the formula).
Having just tried the watch on, and then having her try it on, I assure you, it looks just about perfectly sized on me and laughably large on her.