[IRCA] calulating AM directional ERP when field strengths known?

[Stephen Airy <pianoplayer88key@xxxxxxxxx>]

Hi all...
 
Does anyone know how to calculate the effective radiated power (or whatever is the correct term) for an AM directional station along a particular heading?
Using 1170 KCBQ as an example, I know:
the actual transmitter power (50kW),
the RMS field at 1 km (2,499.50 mV/m - assuming it's operating non-directional - found this in the FCC database)
the field at 1km in a particular direction (3,750 mV/m - estimating/interpolating between the 185° 3559.01mV/m and 190° 3935.08mV/m - actual heading is 187.3°)
 
What's the formula for calculating the equivalent power in kW?
Using the FCC Figure 8 site is A - too slow (would have to go by trial and error, I have several dozen such ERPs I need to calculate, and I really wanted to post a completed bandscan tonight), and B - it's not working correctly right now.
 
I'm working on writing up a bandscan, and would like to post it tonight, but I'm at a standstill, and Google isn't helping me.  I do NOT need to figure out gain in dB relative to an isotropic radiator or half-wave dipole, for example!
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