Generally NOT a good idea to do an override. Why do you think the calculated figure is incorrect? 

I am amending a return that I did not prepare and I am trying to reconstruct the original return as filed and there is small difference in the tax calculation so I would like to override the number and put in the amount that was originally filed.

There is no override. Get it as close as possible. Do the steps to set up an amended return, then adjust the As Originally Reported column in Screen 59, as needed. 

Possibly the copy you have was adjusted by IRS after filing.