water hardness setting with softners

I recently installed a water softner (Ecowater) where the softness of water from system is adjustable. They adjusted output to ca. 100 ppm which they say is normal (by reference the hardness of inlet water is 39°F = 390 ppm). I cannot find anywhere info on what is considered a good compromise hardness given that too low is apparently not good for pipes (according to manufacturer) causing rust and naturally too high is also not good. Anyone with suggestions of what hardness is best for the house (my previous softner did not have an output mixer and delivered <5 ppm - but I had to replace it for other reasons.
Thanks in advance.