Fitting low level re-inforced shower tray?

[longdogs]

Normally when a shower tray bottom is flat I would simply lay it on a bed of mortar but this shower has re-inforcing 'ribs' underneath in a sort of star shape. It's made of some kind of cast stone (not resin) and is relatively light weight. The floor is boarded and fairly flat but not level so it needs to be raised at one end. Do I still lay it onto a bed of mortar making sure that the re-inforced points underneath are supported? There were no instructions supplied.

Many thanks!