Textbook? What kind of textbook? It is supposed to be a very thin book. One that can slide right past your junk and make solid contact with your pubic bone even though your legs are together. Is that how you are doing it? In any case there are other ways of setting saddle height. You can sit on the bike and attempt to pedal backwards using your heels (in shoes). You should be able to do this without any undue rocking of your hips to reach the bottom of the stroke. If you aren't sure go lower than higher. Then when you pedal properly with the balls of your feet on the pedals you should be more or less ok as to height.

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