On the useless tautological sense. If you can't measure CO to within 20%, can hardly measure CI to within that range - then for all practical purposes, the equation does not work.
You may want to read about (lack of) precision in Atwater factors, to the tune of 50%, if you believe CI is easy to compute.
Where CO has a CI input.
This has been known for while. Protein has a thermic effect which can change your CO.
But the equation still works.