Perhaps you should refer back to your ATPL notes. It is in there!
It is also the same time at which you ensure that the nav lights are on!
I tend to fudge it by simply doing a very simple interpolation between sunset at departure and sunset at destination and using departure time to decide on a rough time when it was night.
As for noting when you see the sunset - good idea provided you make an allowance for the aircraft altitude. i.e. sunset at 35000 is long after sunset at the surface (where it counts).
Regards,
DFC