VBA Script not working as Expected

Sorry, it should run when the item is marked as completed in the WIP tab.

So I think I got it working, but I'm not entirely sure how. I tried the same thing before, but now it seems to be working...magic, right?

This is the part that did it:

XX.Offset(0, 11) = XX.Offset(0, 11).Value - BM.Offset(0, 4).Value