Best way to slope shower waste pipe

[lostdiyer]

I’m installing a waste pipe for my shower, with the pipe running about 600mm to the center of the tray, where it will connect to the trap.

I’ve read a fall of 1 cm per every 40cm is adequate. What is the best method to ensure the correct slope has been reached for proper drainage?

I’m using solvent weld if that makes any difference.

I’ve attached an image of the existing waste which I’ll need to extend further.

 

[Madrab]

Min is 18mm per/m up to a max of 90mm per/m - ideal is somewhere in the middle (45mm per m)