No idea what the middle setting is (I believe you set this via the driver), I'd just recommend using it at a value of around 800-1000 dpi, since lower values can cause anomalies with negative acceleration and higher values cause the mouse to jitter.
I have no idea if the G400s is compatible with Linux btw, however I've used the same driver on Windows for my G1, G100s, G400, G400s and G402 without any problems. But it's a pretty fantastic mouse.
I have no idea if the G400s is compatible with Linux btw, however I've used the same driver on Windows for my G1, G100s, G400, G400s and G402 without any problems. But it's a pretty fantastic mouse.