Dehumidifer level in basement
Generally speaking, it's best to keep the relative humidity level between 35% and 55% for hardwood floors (and that's also a comfortable range for most people). You need a humidistat to measure this upstairs where the floor is. Over a couple of days, keep checking and adjusting your dehumidifier to get the humidity within that range.

The actual number on the dehumidifier is irrelevant, and even if it displays actual humidity, note that the humidity in the basement may be different than upstairs. You may also need to adjust as the seasons change.