There are a lot of factors to consider here. Firstly the graphic below is intended to show the latest point at which a driver can see the flag (as shown by the blue flag symbols). The flag points are represented by the blue "smiley faces".
This is because his sight line is way in front of his actual position, and completely different to his racing line. This must be borne in mind when showing a flag, it is no good showing it 50 yards after he is looking in your direction.
The
Largest Area Possible in The Drivers View
Allow for The Direction and Strength of The Wind
Hold The Flag in The Middle of The Longest Side If Necessary, Not Just by The
Corner
Use The Wind to Your Advantage, if Possible Hold the Flag so that the Wind
Displays
it for You
Speak to The Driver, Agitate The Flag to get some movement If The
Situation is Urgent.
Waved
Flag Presentation
Wave the Flag at an Angle That can Clearly Seen by the Drivers
Wave the Flag in a Figure of Eight
The
Speed and Vigour Should Indicate the Urgency
or Extent of the Danger Present
Practice a Wave Before the Start of Racing or Practice Sessions:
Allow for Changes in Wind Direction
Use the Wind, Long Way Down Short Way up,
(or vice versa) is a good way of maintaining a clear signal in
poor conditions
If the Wind is Really Bad, try Anything