Method is this.
Fit tray/bath as normal and seal it to the wall.
Sit board on three 1 mm packers, leave these until adhesive dry, then remove and silicone up. Pump the 3mm gap full and finish with a smoothing tool ( I use the 8mm chamfer which gives approx 5 mm coverage).
The reason for three 1 mm packers and not a single 3 mm packer is so you don’t break a piece of the laminate face off when you remove them.
Slide out the middle one of the three, then press down and the others should come out easily- yes I learnt this the hard way , got away with superglue and custard never knew! But I could see I may not be lucky next time so developed the above method.
To deal with slopes wiggles etc in ceiling and walls. Draw a horizontal line and measure up and down from it, if ceiling bad I take a measurement every 50 mm across. Same with draw a vertical line and measure left and right.
DONT just get an overall height as you will be way off, especially if tray is slightly bowed ( aren’t they nearly all?) or , heaven forbid, the tray’s not quite level!
If you are in a hurry then leave the packers in, cut them flush with the board with sharp chisel and then just press them back a few mil so they’re hidden by silicone ( you shouldn’t have to do this and I would really advise against siliconing the same day as fixing the boards.)