What is the minimum thickness of mud one could safely use on a shower seat (providing I'm using the 4-1 sand/portland mix)?

The seat I'm doing is an extended tub deck into the shower area. I've wrapped a continuous CPE membrane up from the shower pan and over the deck. My slopes and thicknesses are all OK in the pan and curb, however I must have spaced because I only allowed myself a 1/4" mud thickness at the lowest point on the bench. I'm concerned this isn't going to be enough even with lath reinforcement.

I beleive the mud has to be 3/4" which you don't have room for. here is an idea. take the membrane off the deck leaving at the face of the seat place 1/4" cbu on the seat and then use redgard on the top and the face for a water proof membrane that you can tile over. or maybe the pros can chime in with another idea . Good luck