Measure the exact width of the area you would like the blind to overlap. To minimize light leakage we recommend that the blind overlaps the window recess by 5cms at the top and 5cms at each side. (Of course if you want to change this then please do so).
The overlap at the bottom of the blind is a matter of personal choice but please take into account obstacles such as radiators.
Use a metal tape measure, cloth ones can stretch and give inaccurate measurements.