When repairing RV Day/Night shades, the formula for estimating the length of string you will need is:
2.5 x Width + Height = String length needed
(String length needed) x (number of strings in the shade) = Total amount of string needed
It’s fine to measure the window opening and not the shade. The shade won’t be larger than your window. Do not measure your old string, and cut the new strings the same length. The lengths never quite turn out right.
The video below explains how to measure for the correct length and also shows how to restring the shade: