How much of my roof do I need to clear of snow in order to prevent ice damming?
You didn't find a correct answer because there is no correct answer, just like there's no correct answer to "how much does a car cost?", or "what should I feed my pet badger?". It depends on several things, so you may need to rely on common sense and trial-and-error. 


Your climate (typical temperature range)
Your weather (recent temperatures, sunlight, and snowfall amounts)
Your home's architectural type (roof slope, truss heel design)
Your home's roofing type and age (Does it have water barrier membrane underneath? How far up the slope?)
Your home's sun exposure
Your attic's insulation level
Your attic's ventilation level


You can tell if you've done enough when there's no ice forming below the lower edge of the remaining snow on days when the snow on the ground isn't melting. If the snow on your roof is melting off in that case, it's because there's significant heat escaping your home that's not being ventilated away. That's the source of the problem.

Oh, and it's perfectly possible that the answer is "zero feet".