Kind of an old post, but this is how i did it.
1. Add a calculated field to the dataset.
2. Function: OnTime=IIf(Fields!Actual.Value<=Fields!Predicted.Value,1,0)
3. Add a tablix to the report
4. 1st column: Actual, 2nd: Predicted, 3rd: OnTime (the calculated column)
5. Add a summary row, change the expression for the cell to:
Then just change the formatting of the cell in step 5 to Percent. Right-click, Textbox Properties > Number > Percentage > Decimal Places