So if your projections assume 4% growth but in reality only get 3% growth, over 10 years your projections would be off by 14%. Anything projections past 10 years are bullshit.
Here is a some lines from the west wing...
CUT TO: INT. HALLWAY - CONTINUOUS
BARTLET: I don't really need to see the ten-year numbers.
STAFFER: We think it'd be a good idea to take a look at them, sir.
BARTLET: Have the ten-year projections ever been close to accurate?
STAFFER: Depends on what you mean by 'close'.
BARTLET: Within a trillion dollars.
STAFFER: No, sir, but we'd like you to take a look at them anyway.
BARTLET: Okay. Bring me the ten-year projection, a Ouiji board and a magic wand.
STAFFER: Yes, sir. Thank you, Mr. President.
Bartlet enters the OUTER OVAL OFFICE.
BARTLET: What's next?