You could finish your 2017 taxes first. If you filed an Extension for 2018, you have until October 15 to file your tax return.
If you owe less than $1,000 tax for either 2017 or 2018, you will not have a Penalty.
Or, you could leave the amount blank and elect to have the IRS calculate your Penalty, if any.
More info:
https://ttlc.intuit.com/replies/7250569
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