Learned magicians cannot cast "spontaneous" charms. But they can cast charms they do not know, from existing collections. These texts, called formularies, contain "hundreds or thousands of charms". To decide whether a particular charm can be found within a formulary we read:
For each formulary, the storyguide should set a target level for one or more Technique and Form combinations, and set a target Ease Factor from 3 to 9. If the player succeeds on a simple die roll against the Ease Factor, the desired spell is contained within the formulary, unless the storyguide determines otherwise. [HMRE p.85]
This is a bit vague, for three reasons. First of all, we are not really told to which formularies, if any, a Learned magician may have access to; this information is not even provided for sample characters (our troupe assumes one formulary per tradition, available to all characters "in good standing"). Second, we are not given a sense of how the ease factor should be adjusted (our troupe assumes each tradition has a favourite form and technique: the ease factor is 3 for charms matching both, 9 for those matching neither, and 6 for all others). Third, we are not really given a sense of how often or why the storyguide should "determine otherwise" (our troupe assumes it never happens unless a) it is the result of some botch or other "bad luck" situation or b) the fact that a particular charm is missing from the formulary of a given PC or NPC is the initial driver of a particular situation, e.g. a Story stemming from a Story Flaw).
How do you play it?