Most advice about tax comes down to one line: move a fixed share of every payment into a separate account. It is good advice and it breaks in the same place every year — the month you finally get paid for the work you did in spring.
Pick the rate once, then leave it alone
Take last year’s total tax bill and divide it by last year’s income. That fraction is your rate. It will be wrong in both directions across a year and roughly right by the end of it, which is what you need from a rule you have to follow 200 times.
The problem is timing, not arithmetic
If a third of your income lands in one quarter, a flat rate leaves that quarter carrying a third of the bill while the quiet months carry almost nothing. That is fine as long as you never spend from the tax pot in a quiet month. Nobody manages this from memory, which is why the pot has to be a different account and not a number in a spreadsheet.
Check it twice a year, not twelve times
Halfway through the year, compare what you have set aside against what you would owe if you stopped working today. If the gap is more than a month of expenses, change the rate. Doing this monthly turns a rule into a running argument with yourself.
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