I (edward, not the contractor) measured the width of the curb piece to 29 and 11/16". 
There are additional gaps at the ends which are approximately 1/8" so the total 
width is approximately 29 15/16" at the bottom. 

The gap at the top seems to be about29 and 3/4 which is mentioned in the sketch 
as "wall to wall / tight measurement" I'm guessing the glass width + hinge gap
+ normal shower glass tolerance should total 29.75"