In order to reproduce your results from hand mixes of C41 or other color developers, you must be within about +/- 0.1 pH units at 20 deg C with a calibrated pH meter.

In addition, you must be within +/- 0.1 grams in weight of some of the components, and in the case of one ingredient, you must be within about +/- 1 mg (IIRC - I would have to go look it up).

This is mainly for color balance, edge effects and grain as the layers interact with each other and the interaction must be controlled precisely.

This is why I feel it undesirable to mix color chemistry, let alone the fact that most published formulas are only close approximations to the real thing and therefore while they may work with one film, they may not work with others. In addition, testing the results in image structure and color are quite difficult and usually people only eyeball their results. They usually make no direct comparison with the 'real thing'.