A couple of questions re: Drafting

1. How do you specify the booster loadout for a joined metaset?
2. How do you set the number of boosters to be drafted? I had assumed it was the 3 in "3/6/M15", but changing it doesn't make a difference.
2. How do you set the number of boosters to be drafted? I had assumed it was the 3 in "3/6/M15", but changing it doesn't make a difference.