I need to clarify. The example that I gave you was referring to figuring your 2017 tax for use in making estimated tax payments for 2018. This is in part because the current instructions for form 2210 are still for 2018 and not yet updated for 2019.
When dmertz referred to line 15 on the 1040, he was referring to the 2018 1040, for use in doing 2019 estimated tax payments. I want to make sure that I did not confuse you.
If you look at his comments, you will note that line 15 on the 2018 1040 less "amounts from Schedule 4 line 59 resulting from other than distributions and certain amounts that might be included on Schedule 4 line 62" is pretty much the same as the 2017 instructions. Indeed, for most taxpayers, the amount on line 15 is sufficient.
I also want to clarify that line 15 is not the amount of tax that you paid, but the amount of federal income tax that you were assessed. After all, it's quite possible that you might not have paid by the end of the year the total amount that you were assessed (hence would owe the balance on line 22 on the 1040).
In short, your general understanding is correct. When I was self-employed and my spouse was employed, I would count on my fingers and toes and estimate our total federal income tax bill (which, as explained above, includes SE taxes but not the employee's SS and Medicare taxes). Then I would figure out how much my spouse was covering with her withholding - this told me how much I had to pay in estimated taxes to cover the difference. I would then figure a percentage of my gross (my consulting job had very low expenses so gross was as good as net) and every time I received a payment to my business, I would place that percentage of the gross in a savings account to be paid quarterly to the IRS using form 1040-ES. It was a simple system that worked for us.
