As @IRonMaN says.

The safe harbor establishes the minimum amount a taxpayer needs to pay in for ES-tax to avoid underpayment of estimated tax penalty.  The simplest case is that your client is below the threshold for making any estimated tax payment at all.  You would obviously account for whether you would quote your client a fee that would include the preparation of ES-tax vouchers.

Scoping the engagement would be much broader than that.  It would involve looking into the future based on anticipate life event changes, changes in personal finances, timing of income and expenses, to help your client assess whether the safe harbor is the best option as well as how best to mitigate any potential tax and/or P&I exposure.