Need Calculation help.

I have an odd SLA calculation I am trying to build and it just doesn't seem to work.

I have a Completion Percentage. 

Example:

Job 1 uses 5 of 10 allotted days.  It therefore meets SLA.

Job 2 uses 11 of 10 alloted days.  It therefore fails to meet SLA

Job 3 uses 12 of 10 alloted days.  It therefore fails to meet SLA

Job 4 used uses 11 of 10 alloted days.  It therefore fails to meet SLA

Of the 4 jobs, the MEETS SLA percentage is 25%

The calculation I have been asked to provide is an Under Aggregate percentage.

Of the 4 jobs, the total alloted days is 40.  The total days used is 39 of 97.5% of the alloted days.

100% of Aggregated Allocation days is 40, so I need a calculation that is 100% - 97.5% and the result show as 2.5% Under Aggregate.

I have tried:

Sum of Days to Complete divided by Sum of Allocation SLA Days =
DIVIDE(1-
SUM('TABLENAME'[Days to Complete]),
SUM('TABLENAME'[ALLOCATION SLA Days]))

But it isn't coming out correctly.  I know I am missing a SUBTRACTION function, I am just not sure how to write it.

Any assistance would be much appreciated.