The general formula for the heat intensity q is :
q = (e)(Q)/(4 pi R^2), where
(e) is the emissivity of the flame taken equal to 0.0048 x(sqrt of MW of the vapor burnt);
(Q)is the heat generated by the flame, and
R is the distance from the flame.
Codes give different formulas to include factors such as flare stack diameter, vapour flow rates, flare height, etc.
As far as I know, the usual criterion is that the heat intensity at the base of the flare stack shouldn't exceed 1500 Btu/(h.sqft).
Tables show that a figure of 2000 results in blisters after 20 seconds, 5300 in 5 seconds. Values of 3000-4000 ignite wood and vegetation. Compare with solar radiation: 350 Btu/(h.sqft).
I hope this helps.