The usual way is to wind it to give the rated voltage, under full load. Unfortunately, that makes it output a much higher voltage, when unloaded. This was not acceptable for my application, so when I did this, I designed the transformer to output slightly under the rated voltage, when fully loaded.
I ignored the hysteresis and eddy losses (I believe this appear as a resistor in parallel with the primary, so don't cause the voltage to drop much, only increase the current draw) and only took account for the copper losses, which is just the resistance of the windings. In an isolation transformer (1:1 ratio), the copper loss looks like a resistor with a value equal to both the primary and secondary winding resistances in series.
What's the VA rating of the transformer? One thing you can do is look at the datasheets from transformers of a similar size.