Firstly, 3/2 doesn't equal 1.5% but I'll assume that you know that ....
Secondly, is A & B really dimensions, or are they measures ?
Finally, if A & B are measures, then I did this :
if [B] = 0 then "None" else str([A]/[B]) + "%" end
1) Create a calculated field like below,
if [B]<>0 then [A]/[B] end
2) Right click on the above calculated field in the Measures window then click on "Change properties" then click on
Number Format, then select "Percentage"............
As Chris pointed out, there are a couple of issues with the semantics of your question, but I think the crux of your issue is the formatting of the result, right? If so, give this one a try:
IF [Dimension B] = 0 THEN "None"
ELSE STR([DIMENSION A]) + "/" + STR([DIMENSION B]) + " (" + STR([DIMENSION A]/[DIMENSION B]) + "%)"
You may also need to utilize a ROUND() function on the two dimensions divided to get the single decimal, but other than that I think that's what you're looking for.