I think you just need to make the field controlling your label to be a Table Calculation.
Because right now, you are using CNT(MTH151...), it will simply return a count of records as the text label. While you have formatted the display of those counts as 0.00%, they are still just counts of the records in each partition.
You can change that green pill in the Text Label to be % of Total. Table Calculations basically allow you to "involve figures from other partitions of the display" in your calculation. Since you really want pie slice A to be labeled with A / (A+B), you need to allow the records in the B pie slice to "be available" to the records in the A pie slice.
This article might be a little outdated, but explains how the % of Total table calc works.
Thanks a lot this worked perfectly.