Where should curtains end? Drag the hem and see

A curtain's hem is where the eye stops, and a few centimetres in the wrong place makes a whole wall look off. Drag the hem up and down a window wall and see where it lands: pooling, touching, floating just above the floor, sill length, or the awkward middle in between.

Change the pole height and sill height to match your room, try common ready-made lengths, and learn what to measure first, so you pick a length from your own numbers rather than from the shelf.