Mortar bed for bathtub questions

[NTM5]

I am installing a 60x32 KOHLER Underscore alcove tub. The directions of installation say to embed the tub in 1-2 inches of mortar. However, the tub has 4 feet beneath the basin that raise the tub basin about 3.5 inches off the ground. If I follow the manufacturers instructions, my mortar bed will only lock the feet in place but not "cradle" the underside of the basin of the tub.

Do I only need 1 to 2 inches of mortar, or sohuld i put down 3.5 inches of mortar so that the basin of the tub will rest on top of the mortar? 3.5 inches thickness of mortar seems like a lot of mortar to me. What should I do?

 

[breplum]

I don't have scientific knowledge but am guessing that 3.5 inches of mortar might not hold together well vs thinner layer (unless packed in a confined space with lumber on all sides)
You can place extra plywood on the floor to create the thinner space for mortar.
As stated in other posts, we prefer to pack the mortar in by pushing with pieces of wood and make a dam with 2x lumber to act as a block.
Always put down water resistant visqueen or tar paper, and make your mortar mix as dry as possible yet, wet enough to bind.
I've never read anything about "locking feet into place". Those directions are likely generic principles.

 

[NTM5]

I am thinking of putting down plywood, and I think I need about 1.5 inches of plywood. I am going to put down to layers of 3/4 inch plywood. Would you screw the plywood sheets one at a time, or take a larger screw to screw both down? Also, would you suggest using construction adhesive in addition to screws?

 

[breplum]

screw each layer separately. If you have access to torx head screws, they are the best. I am so done with Phillips head legacy poor design.
No need for const. adhesive.