I'd estimate what you need to pay quarterly, and this is what I do, by downloading the latest case study spreadsheet, which is phenomenal, and filling it in as much as you can in columns B, C, G, H, etc. Cell D67 will spit out the total tax liability you should expect, which adds cell D63 for Fed and cell D64 for State. From there I take the Fed and State (Oregon for me) taxes and calculate the percentage of each we will owe based on gross self-employed income.
So for this tax year, I estimated 175k in gross self-employment income (one earner and engineering work so not really a lot of business write-offs, i.e. gross is close to net but ymmv) and 24k in net rental income, married with 2 kids, our estimated Fed Tax for the year = $38542 and State Tax for the 2024 tax year = $14361. With that, I estimate how much of the self employment checks I need to set aside so Fed = 38542/175000 = 22.02% and OR = 14361/175000 = 8.21%. Then I add together Fed + State combined (30.23%) and round it up to 35% to save total out of each self employment check and call it a day (I over estimate because if we earn more than expected later in the year, it will be at a higher marginal tax rate, somewhere around 24% federal and OR's top rate is 8.75%, which combined is just under 35%, I could probably bump it down to 33% but 2% isn't going to make or break us).
When it comes time to actually pay the estimated taxes, I update with the real earnings numbers for the quarter and basically end up sending in 24% of actual gross self employment income for Fed and 9% for OR to have a little buffer and end up with a small refund at tax time. If I end up with a few extra k in the tax savings account, I take it out, which, as you can see I usually do by design - but at least it's sitting in my own account earning interest and not with the IRS. Then I contribute the excess savings to our kids' 529 plans.
