My phraseology would be "Callsign, behind the departing (a/c type) line up and wait behind"....
From an ATC perspective, I would say answer A is correct. We leave it up to you to line up when you think its safe.
If I was a pilot however, I would probably go with B as thats what I would think is safer to do taking into account jet blast / weight categories etc.
I would re-word it though to: b) as above but wait for the preceeding departing aircraft to have COMMENCED TAKEOFF ROLL before crossing the holding position line.
Some do the opposite of your example A and take too long to line up, in which case we will tell them to hurry things along.
Last edited by ASD; 8th Apr 2014 at 18:57.