Its usually a fixed level... whatever level is in the scroll ability. If it has level progression... then it can cast at your level.... if not, then whatever it is set for. IIRC
One of the tweaks in the G3 BG2 Tweak Pack will allow scrolls to be cast at caster level. If you don't have that installed, all scrolls are cast as Level 10 Mage.
my recollection is that Magic Missile scrolls always only give out one missile. That suggests to me that the scroll level is SET and is not effected by your level.
In vanilla bg2 ToB patch, the scroll level is 15. Pierce Magic scroll reduces opponent MR at 15% = 15 levels.
Edit: Actually different spells have different levels. I dont recall summoning the large skeleton using the animate dead scroll. My projected image scroll lasts for 13 rounds.
In BG2 it's fixed at level 10. Since a level 6+ spells can't technically be cast by a level 10 mage, it's cast as if the caster were the lowest minimum level available to cast it (11 for 6th, 13 for 7th, etc.). BGEE seems to uses a fixed level around 6 or so with the same scaling for spells out of the fixed level range.
I never knew this for, I always figured it went with the pnp rules of minimum level to cast the spell unless it was one you made yourself, which wasn't available in BG.
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Edit: Actually different spells have different levels. I dont recall summoning the large skeleton using the animate dead scroll. My projected image scroll lasts for 13 rounds.