So I agree with this idea, but how do you calculate the amount? I'm not asking for a rule, I'm asking someone to calculate me a number for how big AT&T's fine should be.

You'd have to have the rule going in and have the parties present evidence on the amount attributable in the appropriate phase of the trial. Intuitively, the actual number awarded seems far below the minimum plausible if the treble-gains rule proposed were used, but I'll freely admit I don't have hard data to set what the likely actual damages should be under that rule.

Take the gross revenue collected by AT&T for all of the "unlimited" plans that were sold and multiply that by 2.

That's not the formula used when copyright violations are involved...

LOL