Find and read: IRC 163(h)(4)(A)(i), Treasury Regulation (Reg.121-1 and IRC 280A(d)(1) Rev. 71-179, 1971-1 CB 58 and Reg.163-1(b).
They have a magnificent team. These people are always kind and willing to listen to your concerns or issues. Better yet, your assignment is always ready before the time, they usually send you a draft to double-check before they finalize your paper.