My rule of thumb over the years, pick mid range, wait till fridge acquires the setting and readjust up or down from there to personal preference. At home I do the 4 setting and go from there. fridges with 1-10, I start at 5, fridges with a-g, I pick D and adjust from there. I've had some fridges in WDW that don't cool well at all no matter the setting, especially the built in cupboard types, those work better if you leave the cupboard door open to dissipate the cupboard heat generated by the fridge.