I can't figure out how to strip the google trash off the link - google for NASA and 0.84, you'll get the link you're looking for. That number is in a multitude of documents including some MSFC stuff I've got at the house, I thought I had remembered it right and there it is. So I guess they figured that one out a long time back.
edit to clarify: you asked how much plant mass i gave you o2 use
edit on 13-12-2012 by Bedlam because: (no reason given)