What does 100% Paragon or 100% Renegade actually mean?

When discussing the variety of endings to the game, one ending is conditional upon your Shepard being or having 100% Paragon or 100% Renegade.

The ending where you convince the Illusive man to shoot himself in the head.

Does 100% mean that you've filled the bar completely or does it refer to 100% of the choices you made were P or R?

Since at least one option seems to bestow 5 renegade points on you, whether you want them or not, does that mean the ending is in fact only available on a renegade path?

If you've missed a paragon pop up could you end up with too few points or are there more than enough to fill the bar all the way? (I am thinking specifically of the pop up where you comfort Steve Cortez after he puts his picture at the grief wall in the temporary housing area).

Solution 1:

I managed to convince the Illusive Man to commit suicide, and I'm definitely not pure paragon.

Either your sources are wrong, or "pure paragon" doesn't mean what you think it means. (Maybe always do Charm when talking to the IM, and not swap between charm/intimidate?)

Solution 2:

The ending that you're referring to doesn't require you to have 100% Paragon or Renegade, but rather

that you have used a Paragon or Renegade conversation option on him at every other opportunity.

From the Mass Effect wiki:

There are several conversation branches with associated Reputation checks, but the conversation's outcome hinges solely on the final choice. In order to use the final Paragon/Renegade conversational options with the Illusive man, you must have used a Paragon/Renegade options on him at every other opportunity in the campaign, including the meetings on Mars, Thessia and the Cerberus base (which were discovered through other dialogue options.