To create CM against an accounting invoice

Hi, If a transaction was created with accounting rule, say like the rule start from 1st jan 2016 to 1st jan 2017.

What if I want to cancel the invoice, do I need to raise a credit memo with accounting rule ? Or just a credit memo for whole amount?

Another question, there is some tax was applied to actual accounting invoice. How do I add the tax amount in credit memo?

Do I need to raise a credit memo for the invoice line amount + tax?

Thanks

Renish