the correct source is the RUN circuit ( blue wire ) BEFORE the ballast. No matter if regulator or ballast itself.

Which is in fact what FACTORY DID

Coil wire ( blue on earliers, brown on laters ) is NOT the correct source.

On dual field alts, which are the cars with brown wire running to coil, you can splice the source from the same blue wire running to alt field

The reason why the Coil wire IS NOT the correct source, is because the choke will increases load, heating the ballast too, and this CHANGES how the ballast works to feed the coil. Remember the Ballast varies the resistor value to changes the power to coil depending on load and engine bay temp