Is there a globally recommended "hardness" setting for a water softener?
The setting is the "input hardness". The zeolite matrix resin (the part that does the water softening) will nearly completely soften the water until it runs out of sodium. The resin in the tank is good for a certain amount of "hardness" (measured in grains), probably in the range 10,000-50,000 grains. The setting is used to calculate when the resin will need to be refreshed (by measuring the quantity of water delivered, or using your estimated water usage).

If you input too low of a hardness, you'll have softened water for a while, then you will start to transition to unsoftened water. At some point, the unit will refresh itself, and you'll have soft water again.

If you input too high of a hardness, it'll refresh too often, wasting water and salt.

There's no "standard" hardness, it really depends on your water source. As an example, where I am in Indiana, it is about 18 grains/gallon of Calcium. If you have iron in the water, that must be factored into the hardness, usually at a rate of 4 grains per ppm of iron.

If you use municipal water, they probably publish the specifications of the water you receive. If you have a well, then you should have your water tested, preferably using a titration method (the paper strips work very poorly for measuring hardness).