There’s a lot of information online about measuring for your new storms online, and I followed them but didn’t get the results I wanted. I bought Larson storm windows, and Larson cuts an inch off your measurement at the bottom if you order it with an expander. When I didn’t order the expander, the storm window was the correct size! Almost all the websites I researched said to measure the window frame in three locations for the width and height because a window frame may not be straight. It was difficult to measure the window frame on my second floor.
I decided to just measure the original storm windows and take a chance on the second round of storms I ordered. They fit better than the first round. I ordered the storm windows as Tip-to-Tip without an expander using the exact measurements of my old storms. Most people, including Larson, do not recommend measuring your old storms to get the measurements for your new ones. This just ended up working best for me.