That calculation (such as schedule A from the year of the NOL) was supposed to be sent with the tax return.

You must attach a statement that shows all the important facts about the NOL. Your statement should include a computation showing how you figured the NOL deduction. If you deduct more than one NOL in the same year, your statement must cover each of them.

https://www.irs.gov/publications/p536#en_US_2018_publink1000177402