Osmolator Nano 3152 minimum water level?
 

If I place the float switch all the way up inside the DT of plastic rimmed 40 breeder, how far down will the water level drop before the pump starts to fill?

I'm trying to figure out if my water levels will be below the plastic trim some, most, all of the time?

They won't likely be at or above the trim. Be sure to set the controller for the 3 min safety timer for a tank this size as shown on pages 26-29. The pump will kick on when the float is about 2mm above the c clip and stop when it is 2mm from the nut. Unless you can get most of the magnet and float above the trim the water level will be below the trim.