In the latest example of moving the goalposts, you must believe that only flat taxes are fair unless you can come up with the optimum progressive tax that pleases Billi.
But just to the take bait - taxes that make it so the lowest wage earners make enough that they can afford housing, food, healthcare, and furthering their education, even if that means taxing the richest to help subsidize those things. Adjust as necessary, preferably while ignoring anybody who utters the words "trickle down."
There are lots of good examples to look at. Emulate those and adjust, erring to the side of taxing the rich because the consequences for them are less severe than for those that are barely able to provide for their basic necessities.