Mr Geezer, if only it was that easy
There are several hoops that have to be jumped thru, before you can augment the 2 crew to get 3 crew FLT benefits.
I suggest you read the regulatory body publication, and if you are AOC your FOM.
The resting crew member must have a certain standard of rest area, away from the passengers, seperate heating and lighting, and ventilation, as well as the operator must have approval, in the FOM to conduct this exercise.
It is not possible just to say: go down the back and read and chat to the passengers, etc, etc.
Thats why longer range Glfs, can be config with a crew rest area, that complies with the regulations, and then that aircraft can opt for the 3 crew benefits.
I opperated one such grey 3 crew trip,a nd the company determined that the 3rd crew could sit in the jump seat, and as long as it had been rotated back, thru 180o and reclined it complied....
Or the other suggestion was to occupy an unoccuped passenger seat in the main cabin....
Not in my book, but then who was I to complain. The return leg was just 2 crew, and bu$$er the rules.
glf