If you are installing a Site at petrol pump,I dont think there is any Power budgeting needed.A mobile phone does not emit the field density to create sparks on petrol pumps (I imagine the hoses have earthed shielding. The pumps are in a Faraday cage.) How could it happen?
The maximum power that GSM mobile phones are permitted to transmit by the present standards is 2 W (although a manufacturing tolerance is allowed that makes it possible that a particular phone could transmit up to 3.65 W) . In most cases actual transmitted power will be less than this as phones have circuitry to reduce transmitted power to the level necessary for the base station to receive a clear signal in order to conserve battery power(MS & BTS PC). As mobile phones have omni-directional aerials, the maximum power that can be extracted from a mobile phone transmission decreases with distance to an extend that even 1 m away from a phone transmitting at 4W the extractable power will be much less than the 2 W threshold for a IIC gas. Hence it can be concluded that the use of mobile does not present a risk of ignition by means of the radiated energy.
Nice calculations though:-)