Wale,
I recommend contacting flare vendors (Flaregas, Callidus Technologies, etc.) and get there input. The two vendors stated are familiar with industry standards and can provide you the needed information (they provided me the needed documentation).
Another source of information is API (American Petroleum Institute) Standard 521 - Guide for Pressure-Relieving and Depressuring Systems.
With regard to the thermal radiation equations, establish the limit of total radiation/intensity (flare+solar) at the base of the flare and a specified radius (i.e. 150') from the base of the flare. The flare vendor will provide this data (radiation levels vs. distance chart), based on gas moleweight, flow rate, temperature, etc. The flare height will be determined by the radiation level specified at ground level and wind speed.
I trust this helps.
Good Luck
pmover