Accuracy is the key here. If you want total blackout you need to measure accurately.
We recommend that you use a laser measure, Screw fix have a very reasonably priced Laser measure, you can follow the link here. They are also available on Amazon.
For under £20 you can own a laser measure that will give you perfect accuracy, a small investment to ensure your blinds fit perfectly.
For your blind to fit tight you need to measure 3 times for both the width and the drop. Measure as tight in as you can in the corners as this often produces the smallest measurements.
Then please record the measurements on your order form. We will always manufacturer from your smallest measurement to ensure that your blind fits.