I put your statement, boolean(...) in and tried several variation and still did not get it to work
However, I put the statement in an expression pop-up box and the expression:
boolean(/dfs:myFields/dfs:dataFields/d:_1x1Data/@YTDScore <= /dfs:myFields/my:mainstream/my:varGoalScore)
and I also pop-up put it YTDScore and ...