Subtraction with doubles returning zero erroneously
You are not providing the input values in the correct type:

calcPID(0.00, 250, 0.0);


will return 0, while

calcPID(0.00, 250.0, 0.0);


will return 250

UPDATE

Here you can get a working test case: it's running on a different processor, so don't expect to behave exactly as the 8bit MCU at Arduino's core.