Rafal used the following formula to calculate the EV value he expects to read on his flash meter: Number-of-stops-to-right-of-f/1.0 = log_{2}(exp/2.5)

The formula actually requires an illuminance (without time, unit [lux]) value where he puts an exposure value (with time, unit [lux sec]).

Please compare http://en.wikipedia.org/wiki/Exposur...ing_conditions