Many cockpit indicators used for PA calc are on-condition items, and not calibrated at all. Quite funny when you see the ITT/T4/T5 inds calibrated regularly and the Ng/N1 ind is left to its own devices for years. And don't forget that OAT ind as well.
One aircraft automatic PA plotting system that I know of, uses an approximation of the actual graphs, and already has errors built-in. Another reason for using the eyeball/graph job, and another reason that the results from the auto calc are 'advisable'.