....but LDR from your AFM is given from 50' (apart from certain steep approach perf charts or corrected for runway upslope in some fashion) - and that's your target THR crossing height. So LDA is measured from the threshold.
Your deceleration will often begin well before touching down, as you typically will have some Vref+ - corrections applied for wind or similar. These need to be washed off to cross threshold at 50' and at Vref, with further deceleration from 50' till touchdown.
So - the beyond g/s-figure is not used for any practical application on the line (ie excluding test-pilot- and runway design-stuff).