I have a question regarding 1099R forms.

How in the world do you figure the taxable amount for box 2a when it lists the amount as unknown? I know the IRS is supposed to have a table or guide for figuring this out, but even with that, something is not right. I'm using another relative's 1099R form as guidance. They had to do theirs again in November because they entered the wrong amount on their tax forms and got in trouble. So they ended up taking theirs to an accountant. I cannot figure out how the accountant came up with the new figure. Grandma's POA is in charge of the taxes, but I want to make sure he's filling out everything correctly since he's doing it himself.