Amended Return Penalty and Interest Calculation

This is actually about my Pittsburgh local tax, but I think the question could be generalize to all states and federal amended return filing.

 

Here is my situation, I filed my local tax last year on time, and requested a refund for about $1000. However, I find that I forgot to include some of the income that may increase my tax liability by only about $20. Therefore, the correct refund should be $980 instead of $1000. However, since it was a year ago, I am wondering if I need to pay for the penalty and interest for the $20.

 

I guess I should pay for that, but how can I calculate it on my tax form? On the tax form, it still shows that I should get refund of $980. Therefore, it doesn't make sense to calculate penalty and interest on the tax form. I guess what I could do is that I don't need to calculate penalty and interest on the amended tax form, but make a statement and calculate the penalty and interest for the $20 by myself. 

 

Let me know what you think. Thanks!