This comes from our own Jack Leuba.

If you don’t know what that means, you are missing something very important in your mid-to-long range shooting.
Specifically, what the effect the wind speed will have on your projectile in flight in relation to the angle from which the wind is blowing. Those numbers are the percentage of the total wind speed when effecting a projectile in 15 degree increments, or more simply in clock value to the “half hour” level of refinement.
Most of us learned refinement only down to the “half value”, or maybe down to the “quarter value” level, but that’s simply insufficient for what are trying to achieve, and tends to be overly simplistic in the most important areas.
To determine the “wind in effect” value in MPH, simply divide the absolute wind speed observed or measured by the percentage that correlates to the wind angle intersecting the projectile path.
0 degrees (12:00/6:00): No effect (x0)
15 degrees (11:30/12:30/5:30/6:30): x0.25 / 25%
30 degrees (11:00/1:00/5:00/7:00): x0.50 / 50%
45 degrees (10:30/1:30/4:30/7:30): x0.70 / 70%
60 degrees (10:00/2:00/4:00/8:00): x0.86 / 86%
75 degrees (9:30/2:30/3:30/8:30): x0.96 / 96%
90 degrees (9:00/3:00): Full effect (100%)
Example: through observation or measurement, you determine the wind to be moving at 10 MPH. Measuring off of your direction of fire, you determine the wind to be coming from 1:30 (45 degrees off of your direction of fire). 70% of that 10 MPH wind is 7 MPH in effect on your wind deflection. You would them apply a 7 MPH wind correction to your solution.
This does not take into account spin-drift, you’ll need to apply that as usual.
Keep these angles of incidence in mind if you have multiple targets with a lateral change of more than a few degrees. The difference between a 12:00 5 MPH wind and a 1:00 5 MPH wind is enough to bring you off of the left edge of an ISPC target at 1000 yards if you are only accounting for spin-drift. This also illustrates why those “fish-tailing” 12:00 or 6:00 winds can be so frustrating, especially when over 10 MPH.
In conclusion; WIND is the single most difficult variable to precisely account for since we really don’t know exactly what it will be doing when the bullet is on its way, the more precise we can be in our compensation AND observation of result, the more we will learn with each shot. It’s effective to make a best guess and correct the observed miss, but it’s much more informative if we can assign an actual value of effect that the wind is playing onto successive attempts.

-Part 1-
More to come.