So do I. It is fair, easy to adminster, it taxes consumption not savings, it is pro-growth, it makes everyone pay something- no freeloaders, it will tax the "underground" economy, and it eliminates a huge federal bureaucracy.
Your estimate of 15% is probably about right. 17.5% might be needed if we couldn't get at the garage sales, flea markts, hookers and drug dealers, but we will get those revenues, and 15% will be enough.