Subtract values according to condition

Hi all ! How are you?

I have the following problem:

I need to create a Measure that can subtract the value of a Measure and the value of the Benchmark table.

For example

Store Total = is measure.

Store Benchmark = is a value from a table.

Total Benchmark = Store 6 Benchmark (28%) - Store 6 Total (12%) = 16%

I'm blocked with something that seems pretty simple.
Would it help me understand the problem of why I can't subtract it?

Thanks!

Regards!