In addition to all the responses above, after going thru many phases of color management and curves for presswork, keep this in mind:
The coverage on each individual sheet will affect TVI. The coverage varies a little, sometimes a lot, from run to run, so you will find it necessary to adjust SIDs somewhat to account for that.
Also, where the patch you're measuring in relation to that coverage will affect it. I'd say from my experience +\-10pts density is not unusual, sometimes even 20pts to get a better proof match.
It all depends on the specific colors required on each run, some will have high M coverage, some high C coverage. If your 50% Cyan patch is in line with heavy coverage on one job but then in line with little or no coverage on the next, your measured result will vary.
We installed pressSIGN about a year and a half ago, since then, monitoring color and TVI on job after job, I'm amazed at how much TVI varies from run to run and even from the start to the end of a run, from morning to evening, from Friday night to Monday morning fresh starts. I come to realize curves can only be made from a rather large average of jobs and measurements.
So yes, TVI can vary from run to run depending on what the coverage of that specific run is, so you do have to adjust SIDs somewhat. And color management does not mean you will get an exact match in all colors, some will be closer than others at standard densities, so again, if you're job has a color that isn't really close then some adjustment may be necessary. Realize this also, your proof, while totally verified and certified, with an overall dE of less than 1 or 2, still isn't matching every color perfectly either, but will be way more consistent than an offset press, while your press is within standard at dE of 5 or less and has some colors exceeding that. The proof could be off a little one way and your press another but both still within tolerance on a particular color, so again, it depends a lot on the specific color you're trying to match on each individual run.
So you are striving for an average condition that will give you consistent results over time, not necessarily an exact match on all colors, every run, that would be ideal but it isn't realistic. That is a battle we have here, pressman color adjusting every job on press to look better when if he just leaves it alone, at set densities, we get a pretty good "pleasing match" to our proofs and every time we run that job, we will get very similar results. To me, that's the benefit of running to a standard. Keep in mind also that when you start adjusting SIDs away from a standard, you can affect other parameters also, like TVI, and Trap (overprint color), so sometimes when he starts adjusting a job to try and affect a better score in pressSIGN, the overall scores actually go down. At this point though, he's still trying to do that, instead of just running to standard densities, letting me adjust curves periodically, and being consistent. Hey the scores are in the dark green, 80 or higher, sometimes in the 90s and he's still trying to adjust it a little. The problem with that is then I don't get good data from those adjusted densities to create new curves.
There will be variation, there are some colors that don't match nearly as well as others, hopefully most do, hopefully your result is really close to the same every time. Consistency.
