A common mistake in measuring cycling inseam is to use too thick a book. I fear the o.p. is misinterpreting 'large' for 'thick'. Actually you want a fairly 'thin' book that can slide up past your *cough* junk, and nestle in the crotch. The real crotch. Tailors measure only to the bottom of where the boys hang, and the difference for someone gifted (Arnold Palmer enters the discussion) the difference could well be 2.5". I also believe you need to be barefoot when doing this?