Yes, in your scenario, that's probably the best way to go about estimating any tax you might owe -- prepare your return in TurboTax using all of the "good information" you have, then make your best estimation as to what the missing information will be.
For more information on applying for an extension, please see the TurboTax FAQ "How do I file an IRS extension (Form 4868) in TurboTax Online?" below.
https://ttlc.intuit.com/replies/3301811
