Hi, I dont know about the Maths for Heatsinking, just don't do it :
a general rule: if you still can touch it by hand, the electronics will survive ;-?
I think that is somewhere above +50°C for the human skin to be touchable.
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Two other concerns that might help.
TO-220 housing is good for space saving.
However for real power applications, nothing beats a good TO-3 housings or even more modern stuff, . . .
Modern power semiconductors, do provide commonly in galvanic isolation aswell, so that is a benefit even better.
And often TO-3 are better off to be build straigth onto the heatsink.
And isolate the heatsink from the chassis to be bolted on, with some isolating bushes.
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Second, I really don't get it: just adapt the input voltage for any given output voltage towards the needs,
and to create the least possible voltage drop, hence lowest possible series regulator dissipation

I too have plenty of transformers laying around, but also almost never the ideal parameters, regarding power-
output voltages ratios, etc . . .
I just recently came to the simplest idea, of Re-winding each available transformers secondary winding(s)
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And for those who still believe in free energy: there is no such things !
You have to respect the actual transformers power ratings: for secondary aswell as primary winding(s).
a 250VA remains a 250VA.
And from that equation come the voltage and current possibilities

So let us state a 24V secondary for let us say 10Amps.
I needed a 12Vac . . . the secondary windings are always on the outside, and are easy to turn back off, . . .
So I halvated its secondary windings, to get around 16Vac out unloaded.
BUT this never implicates that I can just expect to get at once the double of current ratings

Even one can think always respect the power ratings, especially for the primary that remains the same.
So the 250VA rating in my example, also counts for the primary AC~input.
Hence at 240Vac the max.prim.current remains around 1Amps.
One could think, oh halving the secondary voltage, can give the double of current,
but I am afraid that will not work.
You will need to apply for thicker wires in the secondary, etc . . .