Bally,
You are quite correct, but not sure on the 757 'most airfields' will be well below the max derated MTOW.
Dont know about the loads and fuel that MIA emp is referring to, but with a 233 pax today, LGW - AGP, 15 tonnes fuel, TOW was off the top of my head circa 94 tonnes. Well below 113400 (or as we call it 102,000). Yer the derate was to 53 degrees, the max derate being 57.
Homeward bound with just 180 pax, 13 tonnes, AGP rwy 14 (out to sea), yes it was a max derate, but only just.
I suppose the answer would be to get hold of some performance charts for miami ??