Champion/Challenger Engagement Score

I'm launching a new trigger email but want to A/B test, so I'm using challenger method (first timer). Since I want the test to be equal, do I set the Distribution to 50/50? My confusion comes from the product page that indicates the first percentage will get the test and the second percentage will get the winner; but because this is a trigger campaign it's more confusing to me. Someone set me straight, please!

Also, I'm assuming from what I can find on the Engagement Score criteria that this will score across all subject, open, click, etc?