For me, top tube length is more important than seat tube length. This is one reason I gave my 57cm C-T 1980 Peugeot PKN-10, which I otherwise really liked, to my elder son, who is 3" taller than me. My 1970 Peugeot UO-8 is about 53cm C-T (marketed as a 21" trame), but long in the top tube, so it works OK for me. All of my other road bikes are 55cm C-T with about the same top tube length, and this is my "sweet spot."
1959 Capo with Nervar Star crank and Campag. 980 derailleur upgrades and new Brooks Pro saddle.
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"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069