Old code NBE AE-88 (Spain) instructs to take 25% of the area of the flag, for the global pressure (push and suction)) building-like; quite likely on the slenderness as the code demands an additional x 1.25 should be taken.
I periodically see a standard referenced for the design of flag poles that should give design loading.
I've seen two flags at the same height, one blowing out, one hanging, so I don't know to what extent you can predict the load on a specific flag without testing.