How to estimate tax refund now...?

Estimate what your income is going to be for the rest of the year then add it to what you have and prepare IRS Form 1040-ES which will tell you what you are going to owe.  The difference between that and what you have paid in already is what you need to pay for the rest of the year.

You cannot do that

Use the 2019 forms.  Use form 1040-ES to get the current deduction ($12,400) amount and tax rates.  Most everything else should be the same.  Forms are available at

https://apps.irs.gov/app/picklist/list/formsPublic...

You are wise to do this now and figure out if you are on target, would owe or get a huge refund.

See Max Hoopla's advice.  You should be able to come very close.

The updated 2020 W4 form is completely new and has 2nd job information on it.

If you are working multiple jobs and have not updated your W4 at all the jobs, you should have.

"I got a divorce, and I am having a lot of extra money withheld."

- Filing single will have more money withheld than filing married or HOH.

The US has a progressive tax system.  If you are only concerned about your W2 income (not investment income or itemized deductions):

- add your income.

- remove your deduction (standard deduction or guesstimate of itemized deduction)

= look at a tax bracket chart to calculate your income. 

"but I am not sure where I can plug all those numbers in to get an estimate of my tax liability."

That would be a 1040 tax return form - available on the IRS website.    Tax tables are in the instructions for the form.

ETA:  You just use the form to run the numbers.   Tax rates haven't changed from 2019 to 2020.   You can use the 2019 form if the 2020 isn't published yet.