For predicting, the most popular software is PSUD:
http://www.duncanamps.com/psud2/It's actually quite accurate, though there is a learning curve and you need to know details about the transformer you might be using.
The kind of doubler we use is a cap-input filter, which means the maximum (unloaded) voltage is twice the peak voltage, or 2.8 times the RMS voltage. Under load the voltage will drop, but usually you will see 1.15 to 1.20 times the RMS voltage at the output of the doubler.
