I am in the process of installing the top on my 69 442 convertible but have run into a snag. I don’t know what the measurement is between the rear bow and the third bow. The assembly manual shows it as “X” but there is nothing in there that tells me what “X” is. I have taken some measurements on the old top and it is about 18.25 inches but it is probably stretched out. If anyone can help me with this I would certainly appreciate it.