Perhaps I don't understand your querey, but if you are trying to fly at right angles to your required QDM to the NDB, adding or subtracting 90 degrees will depend upon from where you are approaching the intercept.
For me the main thing with NDB useage is keep a visual picture using the DI & RBI together. If the RBI is saying down to the left, look down to the left on the DI, there's the QDM.
When you know your present QDM, you can visualise where you need to go to find your proposed QDM intercept.