[ars]

Yes, correct. My method was just a shortcut which yields basically the same number because it all atoms of hydrogen, carbon, etc that are burned release the same amount of energy (but you do need to account for the binding energy of the molecule, which my method does not do).

I used it because I didn't immediately find data on the enthalpy of burning plastic in combination with CO2 released.

Plastic is also a hydrocarbon, so the binding energy will be quite similar to natural gas, so my method is sufficiently accurate.