Just a guess, but I think the system will just add up all base AC first, if it's negative, then it's treated as 0. Anything else goes into the SG damage formula.
It says "1% extra damage for each point of base AC on the opponent."
Doesn't mention anything about magical.
This thread is closed to new posts.
However, you are welcome to reference it
from a new thread; link this with the html
<a href="/bboard/q-and-a-fetch-msg.tcl?msg_id=002bmP">SG and negative AC</a>