I think you're dead right DAZ! when I first started breeding my peregrines I was weighing them every few days, a guy told me that whatever the bird weighs at 30 days is roughly its flying weight! it turns out he was miles out mate...
there's alot of differnt things I suppose we all have to take into account, different falconers fly there birds to a condition that they feel the birds hunts best at, some falconers may get a bird flying at a good high weight with alot of fittness and time yet a weekend warrior would probably have to have his bird flying alot less due to its Metabolism and fit