Target has 30 MR with no scaling. You have a void staff, liandries, sorc boots, and 16.2 from precision and mpen runes. So 35% magic penetration and 46 flat magic penetration.
After void staff the target has 19.5 magic resist. When you apply flat pen does this target now have 0 magic resist or negative 26.5?
What would happen to these values if you had an abyssal scepter in the build?