Its best to try and read it at several points going down range to try and get a feel for the changes in the wind due to terrain. If that is not feasible, read it at about 2/3 and call it good. You do this by focusing the spotting scope on an object 2/3's of the way to the target (or at various points along the bullets path)
The mirage will roll or wave in the direction that it will affect the bullets path. So if they are rolling right to left it will blow the bullet right to left. The faster the rolling effect, the more it'll blow your bullet off path. Beyond that, its very difficult to explain how much or how fast equals how much change, etc. That comes from experience.