I don't remember it working like that. My paragon still beat that crime boss in Thane's mission to a bloody pulp despite being a pure paragon.
In ME2 there were a few renegade options that were unlocked with low renegade points, but there are quite a few that only unlocked when you have higher points, and a couple where you need nearly 100% of either the renegade or paragon points available but if you pick some renegade and some paragon along the way, you are screwed.
For example in the scene with Morinth and Samara, I had chosen renegade nearly 100% of the time and still was not able to unlock the option to kill Samara and have Morinth join the squad.
In ME3, it is calculated based on your total reputation points, adding up total renegade plus total paragon, plus any neutral rep points you have accumulated. I got to the highest reputation level rather easily and was able to unlock all the interrupts, despite choosing dialogue options that reflected the character I was playing instead of "always nice guy" or "always rude."