Have a look at this blog entry by Scott Hodge from the Tax Foundation. He writes about the potential implementation of VAT in the U.S., and at which rates VAT should be charged in every year over the next ten years to close the budget.
This is, of course, a rather futile exercise. VAT should be implemented as a part of broad-based tax reform package, with lower corporate and personal tax rates and a simpler system. It is impossible to consider VAT separate from that entire spectrum of tax changes.
However, depending on those lower income tax rates and simplifications, I think that a 8% VAT would be very sensible as a start. It is not too high to create an instant sticker shock, and not too low to be rejected as a serious counterweight against the other elements of tax reform.