What documentation is required by the IRS in calculating business use of a personal cell phone?

[mainett]

Let's assume you are filling out a Schedule C, then using the monthly statements from your cellphone carrier, total up the cost of your cellphone --just the device, not the monthly overhead charges for the service plan -- and multiply it by the percent used for business. Keep that calculation in your notes in case you're audited. You just need to keep a written record or what you used to determine the dollar amounts and reasoning behind the decisions.