I didn't move this to contract because it applies to any sort of print job. Brand of ink should only affect this marginally, though we all know that some matches are tougher than others. Whenever I have doubts with a good match, I get an okay from the client.
You need to get an okay from whoever is paying!
Have them determine whether your mix is what they want, or would rather you fly by the seat of your pants and try to match the last guy. btw, I charge extra on custom mixes that rely on my skill and experience of color chemistry rather than an easy formula following.