Ah the wonders of Pprune -one person poses a question and we get all sorts of interesting spin offs, I am not grumbling I think it is very interesting to see how discussions develop.
I have often wondered how long the Cranford Agreement would last re 09L departures once LHR got extra busy, when I was a kid in my spotting days in sunny Stanwell it used to annoy me that it was always 10L /28L for landing as you only got a really good view when the airport was on Easterlies and take offs were from 10R, mind you we still had the occasional days of using the 23s as consolation.
What we have established for the OP is that aircraft are at all kinds of heights within a fairly narrow band but are pretty much never at 100 ft over the spotting spot.