The best way is to just enter the information into the tax return and allow the program to do the calculations.
Long-term gain is calculated separately on a Schedule D supporting worksheet.
Please see this previous answer from @HalAl
Salary + ST capital gain first. Then subtract your deductions (itemized or standard) & exemptions.
Then subtract that number from $72,500. The difference will be how much of your LTCG are taxed at 0%. The rest will be taxed at 15%.
So, based on what you said and assuming the standard deduction and 2 exemptions (no kids):
50,000 salary
+20,000 STCG
- 12,200 Standard deduction
- 7,800 two exemptions
= 50,000 "ordinary" taxable income
72,500
- 50,000
22,500 Long term capital gains not taxed (taxed at 0%)
30,000 total LTGC
-22,500
=7,500 LTCG taxed at 15%
None of your income will be taxed at more than 15%, because your ordinary taxable income is less than $72,500.
See the "Qualified dividends and capital gains" work sheet in Turbotax for detailed calculations
