If you can tile the tub deck first that is dream. Then your life will be easier.
If you want to tile after then 5/8" spacers work well for 3/8" tile and 1/4" thin-set.
The mortar mix under the tub should be higher than needed. The tub set overtop and squished down to resting position. The tub should sit on it's feet and the mortar mix serves as only bonus strength between the feet.
We use poly to keep the mortar from bonding to the tub.
Just installed one yesterday.
I'll look for the pics.