Start near the center or per Joe Dog's recommendation. You can sometimes squeeze in a slightly larger rear cog by sliding the wheel back a bit, and you can very slightly sharpen the bike's turning circle and handling by sliding it forward, but I just tend to leave it approximately mid-slot most of the time. Also, those who want to put the widest possible tires on a close-coupled frame will probably want to try moving the wheel all the way back.
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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