None of the lateral grooves count at all, only the radial ones.
Some MPSS users have said that grip drops off massively once those lateral grooves are gone, but I've yet to notice that myself nor see more than a handful of people complain, so I'm not convinced it's a huge issue. Certainly I had no issues with the 050A I ran on the 911, front or back.
It's very easy to work out the legal section on a given tyre, but there's no real need to as the wear indicators are there to stop any querying over the exact measurement of the 3/4. As long as you're not below them, you're fine.
I always change my tyres on 3mm, incidentally. Well, excluding 888s as they only come with that in the first place!