Hello, the way you measured sounds like you did it correctly. What you saw was the section that was describing Light Gaps for our shades. When you order a shade as an inside mount we ask that you provide the bracket to bracket width, which is when you measure the inside of the window frame opening from the top left to the top right corner. We also do have the linked youtube video below which shows you how to measure for an inside mount serena or triathlon shade. You can view that here:
https://youtu.be/L9NGdPcxjHI