Filing Extension and Additional Tax Payments

It is possible that I will owe federal and state taxes for 2021 because of capital gains and dividends that i did not expect.  My plan is to do the returns and figure what, if anything is owed, for federal or state taxes.  I want to pay the additional taxes before I file my federal and state returns. 

In the past, I have filed Federal form 4868 and Virginia Form 760 IP and submitted payments with each.  Can I file these forms in Turbotax and submit payments for each? 

I don't want to file the returns before I have determined what additional taxes I might owe.  I assume I can just reopen the return and go back to it? 

Is there any type of worksheet I could use to compute how much federal and state taxes I owe based on income, dividends and capital gains for 2021?   I have nearly all the numbers.  It is a matter of plugging them in.