I have not noticed mentioned in this thread the requirement to use the word 'behind' twice in both the clearnace and the readback.
"Callsign, BEHIND the landing xxx, now departing xxx, line up runway 37 BEHIND and wait."
My understanding is that it is open for the crew to decide if they consider it safe with regard to jet blast when to move in to the runway.
That said it would be dangerous to cross the line before a landing aircraft had passed or before a departing aircraft using an earlier hold to had passed. As long as you remain behind the mentioned traffic.
Rather err on the side of caution though.