Shawn,
Perhaps Devs could have restated his question by asking what effect headwinds have upon takeoff performance and that would have made the question less prone to such concerns you suggested might exist.
Unfortunatley real life refuses to provide accurate weights for cargo....heights of barriers....and always up to spec engines...blades...and the like.
I am sure the test pilot school has courses covering rotor dynamics that would address the topic Dev raises....or would hope so anyway.
Perhaps we could get someone to quote Prouty or the old Sikorsky Blue Book to us.