Since they are slightly different in length I mark the top and bottom belly glass pieces and put them aside.
Another advantage to having the stops on my form is that I can lay out all the marks on the back glass prior to glue up and I know it's not going to slide out of position during air up. I mark center with a square.
Then I mark the shelf at 1 1/8" above center.
I mark the sight window 3 1/2" above the shelf.
I mark center of the width.
I clamp a straight edge to mark center the full length of the glass.
I mark the sight window at 5/32" past center so I can take the final 1/32" off when I radius the window and the shelf.