I was taught to fit bikes by Merle Adams at the shop back in 1971. To this day, that's what still feels good to me, so I stick by it. With a horizontal top tube, I fit a 56cm best, can go up to a 59cm if the bike is rare enough or the price is low enough.
Modern frames (which I'm playing with a bit)? I just stay with that 56cm sizing and it fits pretty well. I find that I'm not willing to fudge on frame size with a modern bike like I will with vintage.
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Syke
“No one in this world, so far as I know — and I have searched the records for years, and employed agents to help me — has ever lost money by underestimating the intelligence of the great masses of the plain people. Nor has anyone ever lost public office thereby.”
H.L. Mencken, (1926)