For each scroll and spellbook you try to write in the game, resolve the random writing chance exactly once, the first time you try to write it when unknown. If that initial attempt fails, mark the scroll or spellbook as not knowing how to write it, so that subsequent attempts will fail with a message like “You don’t know how to write that, so you don’t even try to.” which means they won’t eat up ink either. If you subsequently learn the identity of the scroll or book, you become able to always write it successfully as normal.