ERP = (ed/7.02)^2
e= field strength in uV/M
d= distance in Meters
It is kindof a crap shoot for reasons previously mentioned; you have no idea what the measurement conditions were like. Plus, who knows how they measured the distance to the antenna. Generally, field strength is measured in the main lobe of antenna radiation, which would be in line with the transmit antenna. In many cases, that's simply not possible.
+-RH