Formula for difference in subtotals when subtotal is blank

Thank you, Ibendlin.  I did create a new COALESCE measure to turn blank values to 0, but my above measure still will not calculate the difference when either column has a value of 0.  In my measure above, it will not allow me to bring in my new COALESCE measure. 

For example, when I try to replace ...SUM('Install Hist'[Quanityt]), with SUM('Install Hist'[NEW COALESCE MEASURE]) it will not bring that new measure in.  Is there a way to add the COALESCE measure into my measure above so that it still calculates the difference in values even when one of the values is 0?

I would expect a result where:

If QS4 = 0 and QS6 = 2, the result in Target Remaining = -2

If QS4 = 2 and QS6 = 0, the result in Target Remaining = 2