Fast flow shower waste

[36hho]

I wondered if someone could offer some advice regarding installation of a new fast flow 90mm shower waste.
I have a stone resin low profile tray and fast flow waste supplied with the tray by the manufacturer.
On the installation instructions it states to apply a bead of silicone sealant
To the flange before inserting through the tray,
However I'm also wondering if a bead of silicone should be applied
To the underside of the tray also, or will just the rubber gasket
Supplied be sufficient to seal the underside of the tray.
Sorry for sounding thick but just want to ensure best options to ensure completely watertight before covering over and tiling etc

Many thanks

 

[footprints]

Very often if a rubber gasket is coated in silicone it slips out sideways as the nut is tightened.

Sometimes I do put silicone under bath traps if they fail to seal because the glassfibre underneath is very uneven, but you do have to watch the gasket doesn't squidge out up to an hour after you have tightened, once the silicone is set it will not move.