Mark the center line on the canopy buble. I used a string to measure the distance, along the outer surface, from bottom molding flange to bottom molding flange at two locations, one forward and one aft. Then folding the string in half measured the distance from both bottom molding flange and marked the spot on the top of the canopy. Because of the springiness of the string I ended up with two marks about 1" apart. By marking the middle I got a very accurate center point. Finally using a snap line I marked the center line.
