To answer your basic question about whether your strategy seems reasonable, yes, it does. I say that because you indicated you did some research and used a calculator to come up with the amount.
Of course, I do not know the amount of earnings you will have, your filing status and other pertinent details to your situation so I cannot tell you if this will be enough or too much. If you did not already use our W-4 Calculator, I would suggest running your info there and getting a second opinion so to speak. Since you will be inputting your information, the answer should be pretty accurate. Here's the link to the calculator: https://turbotax.intuit.com/tax-tools/calculators/w4/
With regard to how severe penalties and interest can be, that again depends on several things including how much tax you owe at the end of the year, what you owed and paid last year, when you pay what you owe, etc. Here is the IRS page that discusses penalties and how they are calculated: https://www.irs.gov/payments/underpayment-of-estimated-tax-by-individuals-penalty
It is still fairly early in the year so if you need to make further adjustments, there is still time!
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