Shower Waterproofing

This is my first mud bed...Over all, I'm happy with the outcome and very happy with the Sacrete deck mud from Lowes. It isn't drying crumbly like some of the pictures I've seen in the forums. Unfortunately it came out a little lower than the reference line it was supposed to to be level with. Has anyone else had this come up? I'm wondering if I can mix more deck, mud in the morning and give it a second going over. I'm planning on giving it about six days to cure before I place the FlowFX membrane and start laying tile. It actually has enough of a grade without bringing up the edges to the level line, however there's a bit of hump between one corner and the drain. What are your thoughts?