Vouchers

Tax software is programmed to look at all taxes paid during a calendar year, regardless of source (state / federal). That tax data has to be entered accurately, needless to say. That same data would then be incorporated into any subsequent calculations. If you have significant income derived from sources other than employment (investments, etc.), one alternate to estimated tax is to pay state and federal taxes proactively at the time of any given transaction (sale of equities, etc.). It then becomes a decision as to what percentage of the transaction to devote to federal and state taxes. If one is too stingy, btw, minimal purpose is served with this strategy. Some investors pay 20% federal and 10% state at the time of each transaction, depending on where you live. These percentages are not unrealistic, as any overage gets refunded on April 15th, reducing the burden of deep thought - and a reach for the check book.

Estimated taxes take into account what is withheld by your employer. To figure out your estimated tax, you must give an estimate of your adjusted gross income, as well as your deductions, and credits. This is used to calculate an estimated total amount of tax. At that point, you should deduct the amount of income tax estimated to be withheld to figure out the amount of your estimated tax payments. 

@Steph405 

by default the vouchers for 2025 ES calculated by TT assume that you are opting to pay 100% of 2024 tax thru withholding/estimated taxes (110% if AGI > 150k), and take into account your withholding based on the same amount as 2024.

the other option to avoid underpayment penalty is to pay 90% of your 2025 tax and provide estimates for withholding and other income etc for 2025; to figure this out you need to go thru the interview questions under Other Tax Situations / Form W4 and Estimated Taxes.

Vouchers are based on estimated taxes for 2025, not withholding.