Think in factors, not differences
Being half the answer scores the same as being double it. When unsure, ask whether the true figure could be ten times bigger or smaller than your guess. Aim for the middle of that range on a log scale, which is the geometric mean, not the ordinary average.
Anchor on something you know
Compare the thing to a figure you are sure of. A marathon is a little over 40 kilometres, so a very long river is likely to be a few thousand. Build the estimate from a known anchor and then adjust, rather than picking a number from nowhere.
Break big numbers into parts
For a large or unfamiliar number, split it into pieces you can estimate. Seconds in a day is 24 × 60 × 60. A stadium or building can be sized by rows, floors and capacity per row or floor.
Use the scale for orientation
The slider shows the same scale for every question, from 1 to 100 billion, and its starting point tells you nothing about the answer. Use the labels to decide the number of digits first, then refine. Remember that there is no timer, so take your time and learn from each fact after the reveal.