FW system advantage clarification

Have a question about the FW system advantage. Say if the Amarr have 20% system advantage and the Minmatar have 10% system advantage the math comes out to Amarr having a total of 10% advantage correct? They cancel each other out leaving the difference as the true modifier? Think this is how it works but would love to be corrected if I’m wrong.

Correct, the actual advantage is the difference of the two competing factions.