letterbacon, you almost have it.
all the transformer companies rate their transformers using RMS volts.
and all schematics use RMS volts unless otherwise noted.
so if the RCA schematic says 320 VAC, you tell the transformer guy you want 320 VAC and he will automatically know that you are talkin RMS.
now if the spec for the center tapped xfmr is 300-0-300 VAC, you use 300 volts in your B+ calculations, as the dual rectifier circuit cuts the total voltage of the power transformer in half. one half of the sine wave is flipped over so that you are filtering 2 positive halves of a sine wave. This pulsating DC now has a frequency of 120 hz, which is twice as easy to filter as 60 hz.
a xfmr catalog will also use RMS volts.
the only time you need to worry about peak voltage is when you are thinking about the voltage rating on the caps.
if for some reason the circuit becomes unloaded, maybe somebody turns it on with the regulator, amp and power tubes missing, then the filter caps will charge to peak value, which is rms/.707 as you mentioned.
most folks like to use 1.414 as a multiplier for getting peak value, but either way should give the same answer.
so lets do another example.
lets say you go through your transformer box, and you find a power transformer that puts out 270 VAC on the secondary, from center tap to either leg.
the label was gone but that's what your voltmeter said.
your voltmeter measures RMS volts, unless you are using a scope, which will show the wave, peak to peak.
What would be your B+ voltage after the full wave rectifier and filtering with a cap-choke-cap pi filter?
well, 270 VAC * 1.414 = 382 volts.
so you want the filter caps to be rated at 450 volts for long life and safety.
now the B+ voltage will come down when you load the power supply.
so you do not multiply the RMS volts of the transformer by 1.414,
a typical figure that allows for the DCR of the transformer, the drop across the rectifier tube and the drop across the choke would be 1.2
so multiply your RMS volts by 1.2 and you will have a good guess at your final B+ voltage,
in this example, 270 VAC secondary winding * 1.2 = about 320 volts DC after the choke.
check the drop on the rectifier you are using, there is quite a difference between the various types,
direct and indirectly heated cathodes is one of the reasons.
now the center tapped power supply is mostly found in tube amps, so you have to watch out on your ratings as far as are we talking from center tap to outer legs, or are we talking from leg to leg.
you do not want to end up with a transformer that only has 1/2 the volts you need.
transistor circuits usually use the full wave bridge in the power supply, sometimes they use a bi-polar supply which will then use a center tap,
the single supply is easier to buy a transformer for, since they only have 2 wires on the secondary. so there is less confusion when buying the transformer.
now you do not have the big voltage drop across the rectifier tube as they usually use solid state diodes in the transistor stuff.
so the DC voltage you see from an AC secondary for a transistor supply might be closer to 1.3 times the RMS value of the secondary.
so if you need 24 VDC for a Neve or something, you divide 24/1.3=18.5 VAC.
so you order an 18 volt xfmr and the caps bump it up to 24 VDC.
lots of times the transistor stuff does not use a choke in the power supply, only a resistor, but there will still be a voltage drop across either a choke, or a resistor, probably less across the choke circuit, which is better. resistors are much cheaper than chokes, and much smaller, but you pay a price wth more ripple voltage and less regulation.
