I think what Gysbert was suggesting was that your question is phrased in a very abstract manner - it's hard to follow what story you're putting together, and your title doesn't explain what you're after at all - try to include a reference to your actual question.
If your current week is 17 though, you can get your previous week by using Max(Week)-1 - in order to use that within a ytd setting, you'd want to use:
Sum({<Week={'<=$(=Max(Week)-1)'}, Month>}Sales)
As an example. That would sum your sales for all previous weeks up to your max week, but would still be limited by other selections (i.e. year) but is ignoring your month selection to ensure it's ytd and a month selection wouldn't alter it.
As for your last part "ie 16 x 2000 = 32000 vs 25000 = 78%" - no idea what you mean here, sorry!
