Hi,
Here is an IRS article, you can use the IRS withholding estimator tool, to determine if you may owe or get a refund. https://www.irs.gov/individuals/tax-withholding-estimator
In addition, you can use the TurboTax taxcaster tool, which I think will also be helpful to you:
https://turbotax.intuit.com/tax-tools/calculators/taxcaster/
The substantial understatement penalty is related to not paying the estimated tax payments on time.
Here is pdf regarding IRS penalties: chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.irs.gov/pub/irs-news/fs-08-19.pdf
You can use EFTPS to make these payments electronically. https://www.eftps.gov/eftps/
Here is an article for the due dates of the estimates: https://www.irs.gov/faqs/estimated-tax/individuals/individuals-2
TurboTax will calculate the amount due based on what you have entered as estimates, and the IRS will view their records to see what estimates have been made, then will determine the appropriate balance due for that particular tax year. If you paid an amount due and/or have a payment plan, just make sure you keep a lot out for any notices and make those payments as soon as possible.
