Mud bed supporting Sterling tub question

I have a Sterling Advantage vikrell bathtub and surround to be installed. The subfloor isn't level and slopes down about 1/4" toward the drain (backend-frontend). From the front (drain end), it also slopes about 1/4" back to front with the front being lower.

Anyway, the Sterling installation instructions recommends a mortar bed for such installs, and I think it's a good idea in general to add support.

My question is what do I do as far as supporting the apron near the front of the tub which end up being raised about 3/8" above the floor? Does mortar go under that as well ?

Thanks.
Charlie