Calculating Estimates

Turbotax has a tool called Taxcaster. Google the name and it will come up on the search. You can pay 100% of the tax on your 2018 return so you won't have to pay interest or penalties or pay 110% if your adjusted gross income for 2018 was more than $150,000.....$75,000 if you are going to file married filing separately for 2019.

I'd recommend using IRS Form 1040-ES publication.  You'll see on page 8 of this publication a worksheet for calculating your estimated tax payments.  But since you also have self employment income, you'll need to first complete the worksheet on page 6 before completing page 8.

It's a bit complicated, but it's the most accurate way to estimate your 2019 tax payments.  I'm linking the publication below.

https://www.irs.gov/pub/irs-pdf/f1040es.pdf