As far as I know, there are only two occasions when you need more than 10 Charm points to pass the check. First one you encountered. Second one is paragon check with Saren during The Final Battle, but in this case (fyi, renegade check also require 12 points in the same situation) you need 12 only if you didn't use Charm/Intimidate on Virmire. If you did that, you need only 9 points.
That's what I thought too, I just couldn't remember the situations where I'd need it. I've just got back from Feros and submitted my report. Now ranks 11 and 12 are unlocked and I have 11 charm points. You must get one of the 4 'free' charm points after Feros...
I'm off to Noveria now, hopefully I'll get another charm point 
EDIT: I misunderstood. I totally forgot it works on % Paragon etc. I thought that because I got one of each persuasion for becoming a spectre, the other 3 points were mission based also. Am I right in saying it's basically a point for spectre, and something like 25%, 50%, and 75% paragon?
I'm currently at about 80% so I must have coincidentally passed 75% during Feros.