Using flexible upstand - which side do boards go?

[Oakamoor]

I'm using a flat-topped resin shower tray, the instructions say to use a flexible adhesive upstand so I have bought a classi-seal one.

Backer boards are wedi, I'm attaching to brick wall using metal dowels after dot and dabbing onto the brick.

What I'm not quite sure on is how far to run the wedi board down the wall, should this go to the floor with the shower tray and upstand butted up (and adhered to the upstand?!), or should I drop the boards on top of the tray, in front of the upstand? I'm not sure whether dropping boards in top of the tray will mess with the fitting of the shower frame?

I've asked this question elsewhere and get conflicting answers!