If audited, you must have a “reasonable basis“ to show the auditor how you determined the percentage of business use and the percentage of business expense. I do not know of any specific guidance on what constitutes a “reasonable basis.“
I am not aware of any tax document that says that you don’t need to provide any kind of proof. In fact, I was reading a recent tax court case for other reasons, that concerned two professors who, among other things, had deducted their computer costs and Internet costs because they used them for writing a book. The tax court ruled that since they offered no proof or justification of business use, the deduction was disallowed. The tax court did not offer any opinion on what would have constituted reasonable justification, only that since the professors offered no justification at all, no deduction was allowed.
The other concept that is clearly in tax guidance is that your records must be contemporaneous. That is, your records must have been completed close enough in time to the event being recorded that your memory is reasonably accurate. (You can’t, for example, reconstruct your business mileage log in April for the previous tax year from memory.)
There may be an app that you can use to track your computer use. Certainly, when my wife at the time worked for a large accounting firm, there was a program on her computer that she used to track which clients she worked for at which hours of each day. Or maybe a memo to your self written once a month with an approximation would be good enough. I just don’t know. It would depend on the goodwill of the auditor, and maybe on audit manuals that are internal to the IRS. If you want more specific advice, you will have to see a professional tax preparer.
