There are more qualified people in that department than me, but basically you should aim for the graph to get as close to the red line as possible with crossing it, or just very slightly crossing it. Here's a little more info: https://forum.sector3studios.com/index.php?threads/ffb-updates.4420/
As Christian pointed out, that red line is where the FFB starts to clip. Personally, I have tried a lot of different FFB strengths while testing, and found that letting my wheel clip (where the green line goes over the red) gave me the best feeling. Strong enough to get rid of some deadzone and give me the weight to my wheel that I enjoy, without being too heavy and losing all detail in the forces. Of course, FFB is largely personal preference and highly subjective, but this tool will give you new information about what your wheel is doing to help make your changes more meaningful and hopefully allow you to better adjust your FFB.
Indeed, Green line should be in average the closest to the red line. If it goes over on pics it's not a big deal, you'll only want to avoid the green line to stay over the red one. i.e. : when you you're in a turn on a smooth surface, this is when you want the green line to not be over the red, but a little bit above. So when there's a bump etc you can feel it. If you're over the red line just by taking a turn, you'll not be able to feel "new informations" because you're already at the max level you're wheel can provide to you. Pics value going over red line are ok (it's hard to avoid them, unless you lower too much the FFB)