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Can somebody tell me how this was calculated, please?

There is an easy classical proof that a diagonal frontier in empty space can just move with velocity at most c/2 (adding horizontal and vertical components of the velocity vector.

Try to proof on your own, spoiler will follow later.

1x1 movement can only be done at c/2, or lower, so that means, that a knightship, with 2x1 movement can only be done at period 6, or higher.

And a camelship, with it's 3x1 movement can only be done at period 8, or higher, right?

Correct. It's bit hard to describe the proof, although it's easy