[Sephiroth]

It depends on what you mean "work to". In the same way as the ideal downstream is 0 dBmv, upstream is 40 dBmv (for reasons I've explained in my sticky on the VM forum).

Of course, plant/infrastructure can't be guaranteed across the board to behave ideally, hence the range of acceptable upstream values. The SH maxes out at 58 dBmv and so anything below that which is stable and produces acceptable upstream performance can be tolerated, though far from ideal because there is no headroom after 55 dBmv.

As a rule, infrastructure is behaving well if upstream power is as much above 40 dBmv as downstream is below 0 dBmv and vice versa. You see this in the various stats posted by peops.

But you're right in the sense that the OP needs to keep an eye on the upstream in the way that I have suggested.