sniper, I'm not positive.
Got the data from here
http://www.taxpolicycenter.org/briefing-book/background/numbers/revenue.cfm
and
http://www.taxpolicycenter.org/taxfacts/displayafact.cfm?Docid=203
I'm thinking its the tax that funds Social Security, Medicare, unemployment insurance, etc.
The main thing I looked at was that total federal tax revenue in 2007 was $2.6 trillion and that's what I set FAIRTAX revenue to equal.
BTW, I ran across this site - lots of details about FAIRTAX that I didn't know.
http://fairtaxfraud.com/fallout.asp