I mean, why not just use the symbol for each mastery, e.g. 3WW / 3MM? Players can only begin with a single mastery on an ability, and most characters aren't going to get more than two masteries, maybe three, unless you run a really long time, so it's not going to get too impractical or silly-looking. I get not wanting this to be the official position, if the system theoretically allows you to keep gaining masteries forever, but practically I think it works. I personally found the notation confusing when I was first reading over HQ, too, so I just... wouldn't use it.