Hey Doug,
20 kilo GT is a VERY different fish to a 60 kilo Tarpon mate.... just the nature of how the feed and fight dictates pretty different tackle in my limited experience...
Its all very dependant on the area you are fishing...
Some areas you can get away with lighter tackle, so long as you have the capacity if there is less foul ground around... other areas, dictate the need for heavy drag settings because of foul ground...
The Calcutta just doesn't have the drag capabilities to consistantly handle 50lb trevally in my opinion... it might handle one or two, but the gearing and internals are just not designed for this kind of fishing, despite being a superb reel... I would use a max of 3 ot 4 kilos of drag on it, with the thumb adding additonal pressure if you need it at times...
Maybe look at something like a Saltiga 20, or similar, or Trinidad 16 or something like that which will certainly have better cranking power than a Calcutta....
Better yet, in my opinion, for casting, a good qaulity spinning reel will see you fair even better...