On Monday, May 12, 2014 11:13:14 PM UTC+12, wrote:
From a random flight of my own (unballasted LS4) I observed I used 750m of grass strip to 200ft height, behind a Pawnee with a 9kt quartering headwind.

Unfortunately you didn't say how long that took, so we can't calculate how much air you went through. It's going to be a good 30 seconds though, which means probably 850-900m on a no wind day. In an unballasted single seater. My 1200m estimate for a glass twin doesn't seem too unreasonable.

And sometimes (not that infrequently, actually) we do a couple of takeoffs with a light tail wind (5 knots?) before changing ends.