At longer distances it is often useful to focus a spotting scope at some intermediate distance, either roughly where the bullet is at its max distance up over the line of sight, or else where specific terrain features such as cuts in a tree line, gullies, etc. occur that might be dominating the wind effect on the bullet.
At shorter distances, watching mirage closely (this is where better optics pay dividends) as well as flags will probably help the most. Mirage boards can make things more readily apparent.
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