Charlie and Nigel,

Well, "better" means better.

I agree that there isn't a very good rule of thumb for this. I wanted to try to have the OP make sure that he left a generous margin, something about double the 1.4X he was going to use, since that leaves no margin at all. (And I really like the idea of having rail-voltage margin, mainly for headroom for transients, but didn't want to complicate it too much.)

Probably the only way to really do it right is to account for the effects of transformer windings' leakage inductances, and their resistances, and the diodes' on-to-off-time ratio, etc etc. My spreadsheet does solve and simulate the circuit's differential equations (2nd order when diodes are on and 1st order when diodes are off) and also figures out exactly when the diodes turn on and off, and takes into account the capacitor ESR (but not ESL) and plots everything as functions of time. But I guess it does not take into account the heatsink losses.

Also, possibly the actual transformer being used would need to be modeled. The one in my spreadsheet is modeled from a real transformer, and is scalable in terms of VA rating, Vrms output voltage, input frequency, and input voltage. But transformers vary and may need their own model, for accurate results. The spreadsheet does include the instructions for measuring a new transformer, and will automatically create a new model from the measurements.

And, I used a particular Schottky diode, for the rectifier diodes. That could be easily changed, too, by changing the diode's equation in the VBA code. The model and the procedure I used to obtain the model are in the PDF file describing the spreadsheet, in the post at

http://www.diyaudio.com/forums/power-supplies/216409-power-supply-resevoir-size-166.html#post3279883 .

Also note that my spreadsheet performs a worst-case analysis, in terms of the output signal. It assumes a constant DC output current that is at the level that the PEAK current would be at, for a sine output at the max rated power.

The spreadsheet's latest version, for Excel 2007 and later, is downloadable from the post at:

http://www.diyaudio.com/forums/power-supplies/216409-power-supply-resevoir-size-167.html#post3287619
If someone needs the Excel 95-2003 version, it is downloadable from the post at:

http://www.diyaudio.com/forums/power-supplies/216409-power-supply-resevoir-size-166.html#post3282260
Cheers,

Tom