From what I understand, Paragon/Renegade checks in ME2 are based on a total percentage of the possible points you could have received up until that point.
For instance, as soon as you land on Tuchanka, all the possible Paragon/Renegade points are factored into your percentage from then on. So if you haven't gotten any of the points on Tuchanka it's gonna end up hurting you. Same goes for the other planets.
That's why I always did loyalty missions ASAP.
Certain decisions are also easier to pass if you're Paragon and some are easier if you're Renegade.
Save often, because if you can't pass an important Paragon/Renegade check, sometimes if you go to the lab and respec your class passive to give you 100% boosts rather than 75% it'll push you over the edge and let you pass. If you're already using the 100% evolution then there's not much you can do.
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Gamertag/PSN-Murphiroth
"Would have liked to run tests on the seashells..."