Just installed this with the help of my stepson, who is a builder.  We measured twice, and as a consequence, everything fit just fine.  The product is designed with some capability of adjustment while installing, but still accuracy is needed.  It took us about 4 hours, and two people are really essential to hold the heavy glass panels in place while the locations of drill holes are marked & things are assembled.  In the end, everything fit beautifully, and we are extremely pleased.