I kind of wing it with my quarterlies. It's never the same amount.
I always can generally estimate how much I owe or if I'll break even, so I don't sweat it too much. When quarterly payment time comes up, and I'm a little lean in my bank account and I have to make choice between paying taxes and paying my household bills, the taxes will wait. I can always scrape it together if I get a tax bill from underpayment.
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