Tax is, in a way, an adversarial relationship with the arbiter at the other end. As with many legally defined interactions, there is some discretion on the part of the person making the decision. I would certainly not want to try arguing for the full tax deduction because this would necessarily presuppose ignorance on the part of the IRS representative. That person would probably not like that presupposition. They have smart people over at the Service. I'd err on the side of caution.