If you were just riding 30miles here and there or an occasional century you could certainly do what Mos6502 suggests. Since this is the long distance forum, fit is significantly more critical than for your average cyclist. You don't want to invest the effort and time required to get to the start of a long ride just to DNF part way through because of a comfort or fit problem that resulted from "guessing" at your bike fit. It is possible you could get it right but more likely you'll get it wrong.