The definitive answer to many income tax questions can be found in the U.S. Tax Court Ruling and Opinions based on cases brought before them.  If you are discussing a deduction of significant impact ask your accountant if there are any Tax Court rulings like, or similar to, your situation.  If the IRS questions your deduction and you can support it with a Tax Court ruling you are good to go.  If your accountant gives you a deer in the headlights look when you mention Tax Court rulings find another accountant.  If you are feeling particularly froggy do the research yourself.  Years ago Commerce Clearing House published a yearly book of Tax Court decisions on a yearly basis.  Not sure if they still do as I have been out of the loop for many years.