For example, I don't have charged bolt spell, but I found Staff of Charged Bolt. What level equivalent is that spell cast at?

According to Jarulf's Guide, section 4.1.6, page 74:
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When casting a spell from a scroll or staff, it will have the same slvl as the slvl you know the spell (including any modifications from items). If you don’t know the spell it will be cast at slvl 1 (plus any modifications from items).
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Thank you.