I don't see why the vertical deflection from bullet spin in a crosswind is limited to subsonic bullets though.
Its not, the difference is TOF, or time of flight. The longer the bullet is exposed to the wind, the more the wind affects it.
What have you guys found to be the most reliable way when reading wind (mirage, leaves...)
All of the above, but I like mirage the best. What we need to understand, is wind is different at different altitudes. Meaning, if your shooting prone, what the wind is doing at 2-3 feet above the ground is not what the wind is doing at the top of the arc of the bullet path.
To see this, use two range flags. One at the bottom of the flag pole, and one 20 feet in the air. You will see, most of the time the flags are indication different wind velocities. May not even be blowing the same direction.
Now mirage is different, you can focus your scope to read the mirage at any place you want to, ground level, 20 ft in the air, across a canyon or where ever (just make sure the scope is NOT focused past the target, that will give you a false reading).
Mirage is not hard to learn, just take practice. Take your wind meter and spotting scope out to the field (don't have to be a range, could be your back yard, a parking lot or what ever).
Look in the scope and observe what the wind is doing. Now take a wind reading. With practice you can get pretty good and accurate.
Same thing with leaves, grass, and trees. Take your wind meter with you on your walks. See what the wind is doing to the leaves, grass, trees, whatever, and check your wind meter to see how much wind it takes to make the leaves, grass & trees do what they do.