If Bill is right: you are asking about Payroll. "Supplemental Wages" (see Pub 15) such as Bonus and Commission (non-routine additions to paychecks) fall under different withholding rules, and the employer would already have done this through the payroll process. Your taxpayer will have a W2. You enter the W2. It becomes part of the entire tax return.

If dd4vols is right: You are asking about being prepared to send in Estimated taxes. Since the business revenue contributes to the entire 1040, the concept of 25% is nice, but might not be applicable to your taxpayer. For estimates, that is based on the entire 1040 tax owed, as projected or as "safe harbor" lookback. Having children, spouse with income, investments, etc, all contribute to the final picture on the 1040. The business taxable amount mixes into the 1040.

Or, it might be a different concept, yet. Right now, everyone is mindreading.