Just a quick add-on to this having spent two days trying to work out how the hell all the options work.

I could not see the attributes/mixed option mentioned anywhere so perhaps that has gone in later updates. My solution was simply to make the fist range have a discount of zero ie

10:0.00, 20:-2.50, 30:-3.00 etc

Note that despite it being marked as a price discount without the minus sign it actually adds the figure! Confusing, as sticking a minus in some of the other boxes like that does not work.

Version 1.3.7.1 btw.