On issues of Cell Phone, Computers and some others, these are just 2 examples. The IRS views that if the expence is used for more than 50% business, then it 100% write off. If you dont, then you get no write off for the expence at all. Business travel in the US is subject to what the IRS calls the "All or Nothing Rule" Just make sure you keep good record on these expences and be ready to explain them if asked.