Whatever tip amount you report, you need to be able to show evidence that that amount is correct. As the IRS says in Pub 531,
Reporting your tip income correctly isn’t difficult. You must do three things. 1. Keep a daily tip record. 2. Report tips to your employer. 3. Report all your tips on your income tax return.
As you can see, just guessing won't be sufficient for the IRS. You need to keep up with this through the year.