Lanefan
Victoria Rules
Even that's counterintuitive, as 15 - 18 = -3, not +3. You skipped mentioning the step that says 'reverse the +/- on the answer given by the formula to get the AC you'll hit'.As for the math, its very clearly: (Die roll + Modifiers) - THAC0 score = AC Hit
Thus, a roll of (13 + 2 for a magic weapon) - THAC0 18 = 3, so we hit AC of 3.
Either that, or you got it backwards and it should be [THAC0 - net roll result = AC hit].