Actually Ward,
Whats the area you have left there in the shower area?
Looking at your shower head location…..It might be best to slope that whole area.
A 4-1 sand mix could be used to fill that area. Shouldn’t be too hard to screed if you go off the shower pan.
4-1 can be used from as little at 3/8″
http://www.mapei.com/public/US/products/1005573-4to1-mud-bed-mix-en.pdf
Then after that sets, you could use the wedi subliner to go over it and overlap pan.
http://www.masterwholesale.com/wedi-subliner-dry.html
And if you used the wedi extension, that would get you even closer to wall.
