I have already finished this game once and currently on my second play through. In this play through I want to side with the NCR. However, I am running a problem with the legion. I really want to kill Caesar on this play though by doing the quest Et Tumor, Brute?, I have gained some trust with the Legion however, it is causing problems with Boone, if I continue to help the Legion I will lose Boone, and he has already expressed his dislike for the help I have given them. The only reason I am helping them is so that I can get this quest and assassinate Caesar.

As of now, I have passed a few medical checks for Caesar, however, I still cant seem to unlock this quest to kill him during the operation. He keeps going on about me getting the white glove society to side with him, and I really don't want to do that.

So at what point will I lose Boone if I keep aiding Caesar?
How far do I have to advance with Caesars main quest before I get the option to assassinate him?

I have already looked on the Fallout New Vegas wiki, and they do not give the prerequisites to this quest anywhere.

Far enough that you can't side with house or the NCR.