Bill Orr's Voltage Doubler R Factor Graph
The text below the figure reads
The full load d-c voltage of an IVS-rated voltage-doubler power supply may be determined with the aid of this graph. The secondary resistance of the transfomer is measured and the R' factor is found. For example, a transformer having a secondary resistance of 20 ohms has an R' factor of about 40. The factor is used in formula 7 to calculate the full load d-c voltage of the power supply.
Formula 7
Eload = Enoload - R' x ( I x R)
Where R' is determined from figure 18
I is the full load current in Amperes
R is the secondary resistnace of the transformer

Voltage Doubler Graph