Previously posted to this board - I have used for years to include the 1st qtr estimate built into the tax extension.  That way if the prior year numbers are off we have extra paid in to avoid penalty and if correct, we carry over the balance to the next year.

Yesterday the IRS Commissioner testified that for this year, an extension payment on 05/17 will not be applied as if paid on 04/15 for the 1st qtr 2021.  Only extension payments paid by today would count.  Therefore you could be charged interest for making a 05/17 payment and applying it to 04/15.  Congress was not happy with the Commissioner on this and on not delaying the 1st qtr estimate and discussed making a law change.