Low Level Headboard Tray and Mortar Vs legs

[Teessider66]

I'm replacing my old shower with a low profile resin tray. I've read up that I need to replace the (dodgy) chipboard floor with 18mm marine ply which isn't a problem, and want to lay it as low as possible so am putting the waste under the floor.

What I have read up is that the tray should be laid on a 10mm mortar base to ensure all the tray is supported so it doesn't crack. However, if I want to raise the tray I can use legs, and there seems to be typically 5 legs, one per corner and one i the middle.

So, how is 5 legs OK, but if laid on boards it needs 10mm mortar?

Supplementary questions...
Does the 10mm mortar raise the tray so there is a gap to seal, or is it all inside the tray edge?

How much mortar do I need for a 1400x900 tray?