Each question scores up to 100 points by the ratio of your guess to the true answer. Double or half scores 70, and ten times off scores zero.
The score uses the ratio, so over- and under-guessing are treated the same: 100 for an exact answer, 70 for twice or half, and 0 once you are a factor of ten away.
A run totals up to 500 points.
See also Why is the slider logarithmic. For tips, read the Closest Guess tips and strategy.