If you leave it blank it will give you the Stimulus payment again on your tax return.
Yes you can see your refund or tax due change when you enter the Stimulus payments. Your 2020 return starts out by assuming you didn't get any Stimulus payments so it gives you credit for the full amount. Then near the end you enter how much you actually got so it only gives you the difference if any. You don't get it twice.
Check 1040 line 30 for the Recovery rebate credit. It either adds to your refund or reduced a tax due. Before filing, You can preview the 1040 or print the whole return
https://ttlc.intuit.com/community/accessing/help/how-do-i-preview-my-turbotax-online-return-before-f...
