Hmm, that's not the way i was thinking of it.
What TC are asking is that you relate the amount of runway needed after the glideslope point on the runway. (I think)
If you imagine a "standard" runway and glideslope configuration, then the perf data in the AFM is based on that. So we can take the air distance (from the threshold crossing point) and the ground distance (from the touchdown point, naturally) and add them together, and compare them directly to the amount of runway from the threshold, and we're comparing two sets of distances both references to the threshold. So, apples vs apples, everything works out.
Now suppose that the glideslope intercept point is 1000ft FURTHER down the runway. In effect, the aircraft approach path slides 1000ft downrange, as does the touchdown point, the stop point, everything. So i can't use the normal landing distance required and compare to the LDA, because there's a bunch of LDA 'behind me" by design.
And I can't just provide a crew with the ground roll, because the aircraft doesn't touch down at the glideslope intercept point - by the nature of a flare, the air distance extends further than that. (Only an "unflared" landing would hit - literally - at that point).
But what i can do is...
If a "normal" runway configuration has the glideslope intercept point say 1000ft down from the threshold, and that was the configuration where I developed my performance data, then I can take the 1000ft as an offset and correct all my landing data by that 1000ft.
So if for a 'normal' configured airport I have 4500ft LDR, which means I need 4500ft LDA, that means I also need 3500ft past the "1000ft nominal intercept". So if I move the G/S intercept up or down the runway, provided i fly the G/S and do the same flare, I will ALWAYS need 3500ft past that point. So as long as I know I have that 3500ft of runway after the GS intercept, I am "good".