I will have height + hardness adjustable coilovers fitted on my car. I would like to know how to adjust the hardness after I put the car the at the right height :

front softer -> more understeer
front harder -> more oversteer

right or not ?

 

It's more complicated than that.

It depends where the understeer/oversteer is happening. On corner entry, exit or midcorner.

Just set it to where it's comfortable. If you autox, get a book take notes and do some testing.