take 100% of your estimated total taxes for 2021, subtract your estimated withholding for the full year. this leaves you with your estimated tax requirement for 2021.
since you want to be at 100% multiply by 75%. this is what you would have paid cumulatively through the 3rd quarter and subtract the estimates you already made. pay this on or before 9/15/2021. you can mail in the payment with voucher 3 or you can go online and use IRS Direct Pay and thus avoid the chance of the payment getting lost in the mails but you do bear the responsibility of entering correct info on the website.
https://www.irs.gov/payments
for the 4th quarter due 1/15/2022 substitute 100% for 75%.
since your payments have been uneven you still might have an underpayment penalty. there is the annualized installment method on Form 2210 you may be able to use when you file your 2021 return to reduce or eliminate any penalty. https://www.irs.gov/pub/irs-pdf/f2210.pdf look at page 4 to see what is required to use this method.
