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.
