In any case, I can only compare performance calculated from 50 ft with LDA from the threshold (and again, still remains the question of those extra feet of the TCH above 50 ft?).
gimme one reason why you do that? LDA is the distance available for a ground run. So it all depends on where you touch down, logically. The differences between RLD-ALD used for certification and actual TD point on a ILS will vary for all the aforesaid reasons. TO tackle the issues both boeing and airbus came out with OPERATIONAL landing distances which are realistic and include margin. However that still doesn't mean you'll hit the tarmac on 850 ft mark, does it? Therefore when you fly an ILS approach you'll touch down further into the RWY up to 1500 ft in extreme case, depends on TCH. When ILS is flown operational landing distance must be equal or less to LDA beyond the GS. The difference between the THR and GS is usually 1000-1300 ft. In other words 1300 ft left behind are of no use to me. That's how I do it. Worthwhile to mention that LAHSO LDA is taken from the THR and can bite one in the arse.
