Not sure if I posted it here, but here's how Centurion sized them.....
ST=c-bb, measured along the side of the ST to the lowest part of the scalloped opening at the seat cluster, roughly equivalent to c-top but not.
TT=c-c from the ST at the top, measured along the TT, to the center of the heat tube, at that level.
The "general" sizes began at "square" 56x56, and increased/decreased by 1cm of TT for every 2cm step of ST.
STxTT
50x53
52x54(always seemed more like 53 to me)
54x55
56x56
58x57
60x58
I've seen slight variations, but every tape measure is different, and every frame is different, so take everything at face value and understand this was 30 years ago.