[OlivierB]

I think I have found the tighter lower bound.
But I cannot determine the tighter higher bound, among the two candidates.

From the tighter lower bound that I have found and the information in the question, it seems possible to determine the tighter higher bound.

But I would like to either properly admit or dismiss the K-1 factor.
Can anybody more advanced on the subject propose an indication, a clue, a line of reasoning ? (NOT the answer)

Thx