"Caster Level", multi-classing and Book of Infinite Spells.

[The Hermit]

Okay, so I just got the Book of Infinite Spells, right? I rolled until I got Spell Turning because, ofc it'd be the most OP of all of them for boss fights, since it gives me immunity to a spell or two of instant-kill spells,

However, Spell Turning says the duration is 3 rounds per level. How does this work for multi-classing, and better yet, how does it work if I don't have any mage/cleric spells (as I am a fighter/thief). Does it default to 0 since I am not a wizard? I'd doubt that, but possible. Does it take the average of two levels? Does it take the highest or the lowest?

[kaiyl_kariashi]

Items that cast spells have a pre-set caster level. The book of spells is I think lvl 10 (like most scrolls), but I'd have to check it in NI to be sure.