What seems to work well is to adjust the gain so that the lower level on the graph drops without pulling down the upper level. This seems to result in a spread around -3 dBFS (the ‘loudest’ you want your strongest messages to be) and -25 to -30 ish dBFS (not far off merging into background noise). The value of gain which achieves that is different from site to site as it depends on the kit, antenna, preamps, how it’s sited, etc.
One way to find your value is drop it a notch or two each day and see how the graph looks and how your numbers pan out. Watch the lower value drop each time, and take note if the upper value starts to drop too. Eventually, after a few days of doing this, you’ll find the sweet spot. Note that during quiet periods at night the upper and lower levels jump around a lot so you just want the busy day time traffic.
From that sweet spot, if you increase the gain you’re making your strongest messages ‘louder’ (increasing numbers of stronger than -3 dBFS messages) which doesn’t help them but can mask fainter signals. And if you lower the gain, you just make all your messages a bit weaker without gaining any fainter signals, and possibly losing some.
It’s not an exact science but that approach seems to be good at finding a near optimum gain.