The best way to be effective against pretty much everything when facing a prepared (i.e. competent MP) foe is to have your hardness around 50%. More or less opens you up to counters. Of course in realistic scenarios 60% or so is probably more ideal thanks to SA being generally more present
What?
Here are the ratios of SA vs HA for some typical units that normally makes up 95% of anyone's army:
Infantry without AT: 90% SA
Infantry with sup AT: 85% SA
Infantry with sup+1 line AT: 80% SA
Infantry with sup+2 line AT: 75% SA
Motorized Infantry with SPG (7+2) and sup AT: 85% SA
AI/Basic Tank division (6ARM+4MOT): 65% SA
Tank division with SPGs (4ARM+3MOT+2SPG): 75% SA
So to be able to reduce damage optimally from your average enemy divisions except very special AT or TD focused ones,
your going to want a hardness of between 65% and 90%.
This is just a matter of fighting properly. You don't invade mountains with SHARM.
It's folly to believe that defending infantry moving at 4km/h ( and relying on dug in to be effective ) will get to dictate how, when and if they get to fight against tank divisions moving at 10km/h, especially at the chaotic east front. In theorycraft sure, but in practice much less so.
Last time I checked mountains didn't move at 10km/h and activly tried to catch your SHARM divisions! ( good luck avoiding them if they did

)