High target numbers for Finesse checks frustrate me in Ars Magica, the required target numbers are borderline ridiculous for a typical wizard, and I see them as a game mechanic seeking to limit the use of certain styles of magic. They should be a way to add depth and risk to an activity, not punish players.
In a previous post about extensions to the Conjuring the Mystic Tower spell I commented about the blisteringly high finesse checks which were introduced in the Covenants and later source books. When talking about casting CtMT:
It has some issues in the expanded game, when the Finesse rolls required are extrapolated upon in additional rulebooks, so that it may (depending on the troupe) have a very high finesse check to perform, and with that the risk of a total waste of vis if the Tower is malformed. And a somewhat troubling clean-up task too.
I am an advocate of hand-waving away almost all those aspects given how infrequently the spell would be cast in a typical game, and how fundamental to a covenant’s growth the effect is.
Not to say that a botched spell or flawed finesse roll couldn’t present an intriguing opportunity…
I stand by the point that Conjuring the Mystic Tower was never intended (when first written) to require a Finesse check of 24+. It is implausible for an NPC and frankly a waste of XP for a player character.
For non-rituals, then sure roll Finesse – I accept that a little more. However any time a caster has the time to stop and plan the way an effect will be brought into being, then they should be able to greatly reduce their risk of failing that roll.
Perhaps the guide should be a +1 to the Finesse check for each day the caster spends working on how the spell will be cast. If that approach isn’t punishing enough then use the pyramid scale from XP to require a massive amount of days to be spent to “buy” the finesse roll bonus.