| >>Thanks! Do you know what the arcana requirement is to cast ritual spells (using a ritual focus)?|
Hard to answer without giving too much of the math away, but it's pegged to the skill difficulty of a min prep cast, with a healthy random. In a perfect world, take the DISCERN hard estimate ("You think you will be ranked a raging godling to begin to cast this spell...") and apply that to Arcana. In a perfect world that will give you a 50% chance at the ritual succeeding. "In a perfect world" because there's a lot of modifiers and randoms involved that's going to muddle it a bit, but it should be in the ballpark relative to the numbers involved (harder spells will have a larger fuzzy range).
"Perinthia's astronomers are faced with a difficult choice. Either they must admit that all their calculations were wrong ... or else they must reveal that the order of the gods is reflected exactly in the city of monsters." - Calvino Italo