Step 1. Measure the distance diagonally from the corners of the blind stops and the exterior casing. If there is a difference of 1/4″ or more between these measurements you may want to tune your storm window before installation (see figure 2).
Step 2. Place a straight edge against the inside edge of the exterior side casing. Bows and humps causing gaps greater than 1/2″ between the straight edge and the exterior casing may require the storm window to be tuned before installation. If your window exceeds the above tolerances, you may wish to order your storms oversize. Oversize storms can be trimmed to fit with the optimal 1/16″ space on each side.