Always measure the curtain track or pole width, not the window. If fitting a new track or pole, this should extend 15cm to 30cm either side of the window (to allow the curtains to be stacked back off the window) and the hanging point (eye in ring or track runner) should be 8-10cm above the edge of the window recess.
If you have a rail with a centre overlap, measure the rail as above then measure the overlap and add the two measurements together.
Measure from end to end – A
Measure from end to end with overlap – A + B
Measure used pole only – C