@LSTAX "May be best to aim to keep the amount due less than $1,000!"

I'd rather aim for keeping the penalty less than $100, because then IRS doesn't bother with assessing it.  Used to be, the safe amount was a $1,400 underpayment.  With the lower penalty rates these days, it's probably closer to $1,800.   A taxpayer who makes four $250 payments -- or even four $300 payments -- has a fool for a practitioner.