"How would dispatchers make their computations" - By referring to the Performance Engineering Manual, refer to
Mutt's post.
It ain't rocket science, for one aircraft that did not have such info in the PEM, I was tasked with writing a computer programme to calculate CI. Inputs are normal FPPM performance, Fuel cost, Non-Variable Overheads etc., a quite straight-forward task
Regards,
Old Smokey