David Tubb... has an excellent book on wind deflection --- along with other shooting accuracy tips.
Tubb...focuses his spotting scope, ten yards short of the target --- so he can read wind mirages.
I doubt that. I don't know of anyone who can resolve focus good enough to discriminate between a perfect focus on two things 10 yards apart at 1000 yards. Even with a spotting scope with a 100 mm diameter spotting scope objective lens. I've tried it with the finest optics on this planet and with near 20-10 vision most of my life, could not resolve focus between two things 10 yards apart at 1000 yards.
I'm surprised if David really put that in print. Well, kind of surprised. Having known David since he was a teenager back in the 1970's and shot many a match with him and his dad, George, David's well known in High Power Rifle competition as one who "stretches" things to gain an advantage. I've watched him adjust focus on his spotting scope every time he gets ready to shoot. If he really did focus at the same place all the time, he would not have to change focus on his spotting scope.
Most match rifle competitors focus their scope somewhere between them and the targets range so they can see it wrinkling across their field of view centered just above the target image. You need to see that mirage wherever its at. Shooting 5 days in a row on the same shooting range at targets from 200 to 1000 yards away, I've noticed I have to change focus for each target distance depending on which direction the winds come from as they ain't uniform across the low hills and valleys around the range. Whatever focus gets you the best mirage is what you use. It can also change from time to time during the day.