I like a 7 percent ratio progression myself. I run 45-42 / 13-15-17-20-23-26 on the Peugeot, and would add a 30 and a 34 for a 2x8 system. I am a big fan of either half-step or 1.5-step, such as 50-42 / 14-16-18-20-23-26.(... -30 for 7 speeds, etc.)
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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