At this point (Black Friday sale), I think it's safe to say their current policy is not to take off more than 50%.
I say this because at some point, I bought Common Sense, as I recall, at 75% off. It is now only 50% off during Steam's Autumn Sale and Paradox's Black Friday sale.
IMO, it was stupid of Paradox not to make this explicit at the time: I'd likely have got some DLC sooner, and they'd have got my money sooner. I assume they guessed that it would hit their brand less hard if it dawned on us slowly that the good old days of 75% off were gone than if they'd publicly and unexpectedly announced it.