Some of that will be mitigated by the larger tax base (food, new housing, rents, internet sales etc.) that now go untaxed in most states. 6-8% may be low for some states, but a reasonable guess without crunching a ton of numbers.I actually don't think states would be inclined to go with the wider tax base because it would make their taxes more regressive. They would have to set up some sort of state-wide FCA.
That makes for a higher rate yet.
According to a chart in Emancipating America from the Income Tax by David R. Burton and Dan R. Mastromarco (the authors of the fairtax bill) the fairtax rate is increased by more than 19% to "pay for" the FCA.