"I didn't ask how much you knew, Mycroft." The words come out like grit, and he has to bite back pettiness. "I asked how much he told you."

Because it's not a subject he and John bring up with each other. Breached it once, left it incomplete, silently agreed to never touch it again. Better that way. (Pointless to talk about when neither of them know the ending.)

He needs to know how much John was willing to share.