Thanks, Aleph. You're right; I was making it too complicated. :approve:
For closed tubes, ƒn = nƒ1
So, in response to your question: ƒ2 = 2f, ƒ3 = 3f, ƒ4 = 4f, and so on.
With the 400 Hz difference between successive frequencies, and knowing that even harmonics are skipped in...