Is “that of” used in an appropriate way?

Question

Could you please tell me whether I use that of in an appropriate way or not?

Here are the results of the calculations. That of calculation number four is pretty difficult to get.

Answer 1

In this case that is a demonstrative pronoun standing in for the phrase "the results of the calculation". So it is fine as it stands, except for one modification

[The results of the calculation] of number four [are] pretty difficult to get.

Note that the number of the copula changes from singular to plural, is to are, since the antecedent (results) is plural.

Edit:

In the case I cited above, it is the copula that would change. In your case, splitting the thought over two sentences, the demonstrative pronoun that would change number as well:

Here are the results of the calculations. Those of calculation number four are pretty hard to get.

This is only if number four would have "results" instead of "a result"; it's a minor point, but if you mean

The result of calculation number four is pretty difficult to get.

then the singular "That of ... etc." would be fine. Whether or not there is a single result or multiple results for a single calculation is not clear from the example.

Answer 2

"That of" used when you are saying which person or thing you are referring to:

The voice was that of an elderly woman.

So, I think your sentence is correct.