It wasn't clear to me until I ran the numbers and then reread the text with them in mind. If I was writing a rule from scratch, I would probably base it on the optimal number with the assumption that at some point you just say "would you guys just get out of our way so we can work?" That would be more intuitive.
Then use a formula like N - 3*((N-1)/Optimal)
For an optimal number of 2 that yields 1 or 1.5.
For an optimal number of 5, that yields 1, 1.8, 2.6, 3.4, 4.2.
For an optimal number of 10, that yields 1.9, 2.8, 3.7, 4.6, 5.5, 6.4, 7.3, 8.2, 9.1
And if there is effectively no limit on how many people can help, then the result is essentially just N, which is fairly intuitive.