To plan for next year you can go though the Estimate section or in the Desktop program there is also a What-If worksheet. And in Desktop, you can save your return with another name to test in. Go up to FILE – SAVE AS.
To plan for next year in the Online version you can go though the Estimate section. Click on Add A State to let you back into your return, Be very careful not to change any of your actual current return. Or you can set up another account to test in. You don't have to pay unless you want to print it out. You can use the same email address for 5 accounts.
If you have the Desktop program you can do a What-If worksheet. Go to Forms Mode, click Forms in the upper right or on the left for Mac. Then click Open Forms box in the top of the column on the left. Open the US listing of forms and towards the bottom find the What-if worksheet. It's right under Estimated Taxes.