As near as I can tell, Paul Ryan and his staff just took the CBO projections that ended in 2019 and drew a random line, extending upward at about a 45 degree angle, until 2080. There's no real attempt to make it look scientific.
That's a 70-year projection no less, to make the Democrats look much worse (if you extend the obama line for the same amount of time, the government is spending 90 percent of GNP at the end). I can barely find a single conservative blog touting the thing, which tells you something. And the spending freeze on discretionary items right now seems clinical. The GOP says it wants fiscal discipline, after spending like Sarah Palin at Saks for eight years. But it seems to have no idea how to accomplish it in fact.