1. Yes, your understanding is correct.
2. You can use IRS worksheet or TurboTax tax calculator to estimated your estimated taxes. Remember to divide yearly estimated payment in two since only two quarters are left.
https://www.irs.gov/pub/irs-pdf/f1040es.pdf
https://turbotax.intuit.com/tax-tools/calculators/taxcaster/
3. If you have more gains later in the year, you have to recalculate your estimate payments for last quarter.
4. Here is a link to both to make estimated payments. There are several methods you can use.
https://www.irs.gov/payments
Hawaii Tax Online
5. Yes, if you miss a deadline, you can still pay estimated taxes but there might be under payment penalty.
6. Both IRS and state will charge under payment penalty based on certain factors. Here is a link for more info for Under payment penalty.
https://www.irs.gov/payments/underpayment-of-estimated-tax-by-individuals-penalty
https://files.hawaii.gov/tax/forms/2022/n210ins.pdf
Hope this answers all your questions.
