You can't reference a calculated field, you have to recalculate the value. You need something along the lines of:
PerCentIs: (Me![MyControl]/Sum(Me![MyControl])*100
However, having not tested this you may get erroneous results if the records load incrementally.
An alternative solution is to build a totals query for each 'subject group' and join that to the original form query. You could even calculate the percentage in the query as a control and add it to your form.