Shower tray not completely flat, but not sure if that is okay!?

[smallsteve]

I have just received a resin capped shower tray and as best as I can tell, one corner at the back (i.e furthest away from the trap), is 3mm lower than the other corners. To be clear, this is across the top of the tray over which the tiles will be situated, although 35 mm of the edge will still be exposed. Is this acceptable? Is there a maximum accepted tolerance for flatness?
This is the second tray I have had from them, with the first having the same problem but in the opposite corner. It makes me wonder whether I am being too fussy.
TIA

 

[king of pipes]

3mm is acceptable in my book add a plastic packer under the low point dry fit the tray first before bedding down . Kop