Measuring windows with a break
Prefer using metal tape for the accuracy if the measurement. You need to measure the width at three location. Measure at either ends and in the middle use the smallest measurement for reference. For roman and roller blinds you have to deduct 1 cm for smooth operation.
Similarly for the length, measure from the top to the recess at three points and note the lowest measurement. Again for vertical blinds reduce the measurement by 1 cm.
Measuring windows without a break
Measure across the width and add 10 cm to the measurement for to allow 5 cm overlap on each side for complete blocking of the light.
Measure from top to bottom till the height you want the blind to fall.