Lower the blind so that it's completely straight. Take a horizontal and vertical measurement of the shade. Add the two measurements together and make note of the total.
Measure from the top corner of the shade to where the tassel should hang. The tassel is located on the end of the string where you pull to lift the shade. Add this measurement to your previous measurement.
Add 18 inches to your total measurement and round up to the nearest foot. For example, round up a measurement of 47.8 inches to 4 feet.
Repeat this calculation for additional strings. If the string of the day-night shade crisscrosses in the back of the shade -- common in RV day-night shades -- double your final measurement.