Tank Venting Calculations

hello,everybody:
how to select the wetted surface of a low-pressure storage tank when itself catches a fire rather than it is exposed to a pool fire ?
when we calculate the fire water demand or tank breathing valve's venting capacity of a low-pressure storage tank with PV valves on the roof,we will decide the wetted surface of the tank.but in all of the standards or handbook,such as API RP 521,API 2000,they say:the up-limit of the wetted surface normally is 25 feet(required in API RP 521-1999 clause 3.15), or 30 feet(required in API 2000) ,the part which is higher than that up-limit may not be protected or may be not calculated when calculating the breathing valve's venting capacity.All of the up-limits are based on the assumption that the tank are exposed to an outside fire or pool fire,the pool fire's flame can not impinge higher than that up-limit.
We ignore the conflict of the value(25 or 30 feet) of different standards,we talk about why all of the standards consider the up-limit based on the assumption that the tank are exposed to an outside fire or pool fire(it means to calculate the venting capacity of adjacent tank rather than buring tank)?why not consider the burning tank itself?if a low-pressure storage tank catches a fire at its low-low level,and this tank's height is higher than 30 feet ,how to select its wetted surface when calculating breathing valve's venting capacity?
in API 2000,table 3B footnote a,it says"Engineering judgment is to be used in evaluating the portion of the area exposed to fire."
so,for the buring tank,the whole tank height shall be considered.
but,why all of the standards consider the tank venting capacity based on the assumption of exposing to a pool fire?why not consider the buring tanks' venting?