Extracting this code I came across (field names have been redacted):
(CASE WHEN (a.Field1 - b.Field2) > 0 THEN a.Field1 - b.Field2 ELSE 0 END)
Logically not wrong, but the number of mathematical operations can be cut further to increase processing speed.
Zoom in here
(a.Field1 - b.Field2) > 0
There is an arithmetic operation and a comparison operation here. This can be reduced to a single comparison operation with no loss in logic. The arithmetic operation is only executed when the expression is true, otherwise the literal 0 is returned.
(CASE WHEN (a.Field1 > b.Field2) THEN a.Field1 - b.Field2 ELSE 0 END)
Of course I may be wrong as I have not done any benchmarking. 