re: Tax Filing Extension

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With estimated taxes you avoid a penalty if you pay 90% of the current year's final number or 100% of the previous year's tax due whichever is lower by the 15th. It is shorthand for quarterly payers so as long as you pay 100% of the last year's total by the 15th you avoid any penalties but you could have a big check to cut about a month after your 3rd quarterly for the year. Been there done that but I knew it was coming. IIRC withholdings payers don't have the 100% of last year to rely on I think they have to pay the 90% of the current year. Don't quote me or rely on that.

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Not sure if I’m missing the point of your post, but the 90% thing refers to estimated tax underpayment penalties, not late payment/interest penalties, which is the consideration at extension time.