The way the checks work is you have to have a certain percentage of the total Paragon/Renegade points you could have gotten up to that point.
So say you've done enough stuff that, choosing everything right, you could have gotten 1000 Renegade points, then you might need 900 of them to pass a certain check, but if you could have gotten only like 500, then you'd only need 450 of those.
It's nice in that it means you don't have to go mining for Paragon/Renegade points before doing certain tasks, but it makes it a lot harder to mix it up and still meet checks, you pretty much have to be one or the other all the time in order to make the hard checks.
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Goodnight Austin Texas, wherever you are!