headphones on speaker out

usually it´s done with a series resistor but i don´t know how to calculate that exactly.(in a grundig amp it was 560ohms/1W)
but that´s definitely not a high-end solution.
for a better sound better build something with a good opamp(opa134...)

i wanna add a headphone jack in the same way
so i´d appreciate a hint for calculating the correct value for the resistor