I’m renovating a 1900 Victorian. In the bathroom, we used a 42”x60” TileRedi shower pan.  I recently tiled the entire shower, including the base.  When I put water in the pan, it doesn’t drain correctly. There’s a puddle about 3/16” deep (see the reflection in the photograph). From what I can tell, there are likely two issues:
1) the base was not set exactly level. I have confirmed this with a level on the curb. (I didn’t set the pan- my framers did)
2) under the tile in the pan, I suspect that the thinset is just slightly thicker closer to the drain. If I drop a 2’ level on the pan from the trough to the middle of the pan, it shows “level”- not sloped down to the drain as it should. But the water doesn’t pool any higher up the pan, hence my conclusion. (I set the tile, so if this part is true, it’s my fault)

Here’s the question- I want to just tile over the existing tile to correct this. I’ve seen a number of articles that say this is OK, (incl in FHB). What they don’t address is the correct way to adjust the slope in the base. Is this something I should do with thinset? A bit of mortar? Also, any specific ways to make sure I get it sloped correctly this time?

I only have about 1-1/2” to play with here, otherwise my “curbed” shower starts to become “curb less” at the shallow end.

Photo shows about 48” of the pan – note the reflection of the window. The puddle stretches from the trough drain to about where the piece of colored plastic is sitting.