BCB,
When I first set up the spin drift calculation modification to the Precision Ballistics~Ultimate software, the spin drift value was simply added to the right or left wind displacement. Even though I was the one that set it all up, I was constantly commiting the same error as you describe.
My primary objective when programing the Precision Software is to make the applications as user friendly as possible so, after a lot of frustration, I finally set up the trajectory table with both the spin drift and wind deflection values. The trajectory calculation values are much less confusing now.