To start, you need to understand wind, and its affects on bullet, travel. If at all possible never tune in a head wind or a tail wind, as these two winds are the most difficult to read, and affect bullet travel the most. These two winds not only introduce vertical, but they also have some horizontal built into them, do to the barrel twist. So with that said a head wind will send the bullet down and to the left, and a tail wind will send the bullet up and to the right, depending on the speed of the wind will very how far the bullet moves. Also head winds and tail winds are difficult to see movement on wind flags, as the flags are pointed towards or away from you, and subtle movement is hidden behind the tails or if you have propellers. Here is an article that may help
http://www.uslink.com/~tom1/windvector.htm
Others have said to tune on the calm days, and well that is not the answer either. Shooting in the calm can be one of the most frustrating things on earth. Calm days can produce affects unreadable by wind flag, and mirage, and make you scratch you head or worse off kick or throw something. If I had to pick between two choice on of them shooting in rain with a 10 mph cross wind, or shooting a nice sunny day that is calm, I would choose the rain and wind any day.
Now going on with that said if you have the chance to pick wind a nice day that has a consistent cross wind. The thing is I have had few days where I could pick the weather and wind conditions, and most times I just have to go out and deal with what I got. This is a better thing, as you learn a lot about your rifle and reading conditions.
Two last things one if your tuning a 600 yard gun, don't waste your time shooting at 100 yards, if you have the ability of longer ranges. Preferably tune at 300 yards, if you don't have it 200 will suffice. Don't always pick the smallest group, but look for the group that is more wind tolerable. (Check out Tony Boyer's Book for explanation.)
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