If it's reading 2% low, then increase the setting by 2% (multiply whatever it is by 1.02). Not real hard. Remember, it's just counting revolutions, and every time it counts one, it "gives you credit" for having rolled forward whatever it has been told the circumference of your tire is. If it's reading low, that means the tire is bigger than it thinks it is and you should increase the setting. If high, then it thinks the tire is bigger than it really is and you need to reduce the setting.
I don't think that rolling it out is all that accurate, I use it as a starting point, but then I put a hiking GPS on my bike and ride 20 miles or so and reset it to be accurate to that.
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Work: the 8 hours that separates bike rides.