You will need to target line 24. Your total tax and divide that by 4. Line 37 is line 24 minus any withholdings and credits you had. Unless you are 100% sure, you will have the same withholdings and credits, it is safer to go with line 24. Now, if you have a regular W2 job along with self-employment income, you can deduct what your employer will withhold from line 24 to decide how much you pay.
As for paying without the Magic 8 ball. You can absolutely do that. Depending on what type of job you have, you could use something like QuickBooks self-employed to track your income and expenses, then at the end of the quarter it will tell you how much you owe with what you made.
You can also manually do it by using the amount you make in a quarter, then figuring out your regular income tax on that amount plus 15.3% for your Self-Employment taxes. You would want to use this chart to determine your normal rate.
For example, if you earn $25,000 in the first 3 months of 2023. You would need to base your numbers on $100,000 when calculating your taxes. So your tax for the year would be $14,767(Ordinary Income Tax) +$15,300(SE) =30,067.50. You would then divide that by 4 and send the IRS a payment of $7,517 in April of 2023.
But then if the next quarter you earn $50,000, you would pay your taxes based on either $150,000 ((25k x 50K)x 2) or $200,000 for the year plus your SE taxes. Then in the third quarter say you earned $35,000 and 4th quarter you earned $20,000. For the third quarter you would base your payments off of $145,000 ($110k earned so far plus a guess of $20K for the next quarter) Your 4th quarter payment would consist of either a catch up or lower payment.
Basically, as long as you are making the payments based on what you earned in that quarter, you will be able to avoid penalties and interest as you can then file form 2210 to show your payments were based on the quarter you paid.
So, in reality, there will always be a little guessing, but you can file based on what you earned for the quarter you are paying.
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