Which means the rectified voltage would be 30 x 1.41 = 42.42V, plus a few percent for transformer regulation (increase off load)
A high quality 300W transformer probably has a regulation factor of roughly 7% or so; eg. the voltage with no load will be something like 7% higher than the voltage at full load.
The output voltage will also depend on how well matched the supply voltage is to the transformer primary fvoltage; eg. a 230V transformer fed from 240V (which is perfectly OK) will be a few percent higher again.