I did something akin to this, I'll tell you what I did, see if it helps you at all.
I used an MDX statement to calculate an aggregated "worst value" for each level of the hierarchy I was using. This was needed b/c in this case, the scale was 1-4, with 1 being "best" and 4 being "worst", and I had trouble getting any sort of automatic rollup calculations to work. I didn't need an average, I simply needed the "worst" (highest) value in each case, to be displayed.
Then, in my KPI indicator, I set the Target value to 1 (Fixed Value), b/c 1 was the desired target (1 being "best".) I set my indicator as "Decreasing is better" and Banding method as "Band by numeric value of Actual". I set the threshold values as: Best = 1, Threshold 2 = 1.1 (just past 1), Threshold 1 = 2.1 (just past 2) and Worst = 3 (b/c both 3 and 4 were bad.)
This worked very well for my Scorecard and KPI rollups.