And let's not forget the input voltage too. This should be measured right at the radio, not at the battery! The reason is, the power cable which comes with the radio is barely adequate voltage drop wise.
Further, it has been noted in reviews of the radio that the power out is a few percent lower than advertised.
In any case, the only way to be sure, is transmit into a dummy load, with a known-accurate wattmeter. Even then, 5% is as close as you're liable to get.