[radiomobile]

The simple system as described works well and experience shows that results correlate well with transformers with known ratios.
There is negligible current flowing in either winding unless the transformer is faulty, eg shorted turns, so there is significant 'sag' of the 1 volt supply, and if greater accuracy is required simply divide the output voltage by the input voltage (nominal 1 Volt) as measured on a meter. As a meter draws negligible current a single meter will suffice, similarly as no current is being drawn the resistance of the windings, (ohmic losses or copper losses ) can be ignored. At a frequency of 50c/s phase shift errors can also be ignored. Although the comments are technically correct there is a great danger of over-thinking what is basically a simple and effective procedure