made to measure
I'd strongly recommend fitting the curtain pole or track before measuring up, so that the finished curtains are the best possible fit. Use a metal tape to measure, and double check all measurements. Your track or pole should be fitted 15 cm above the window, and extend by 15-20cm on each side (or 30cm for tab top and goblet styles).
For a curtain pole, measure between the finials.
For a curtain track, measure the entire length.
For a split curtain track, measure parts A + B (see diagram) to allow for the overlap.
Choose what length you want your finished curtains to be. Take measurements from the top of the track or pole. I'd suggest the following:
2. measure up
For sill length curtains, measure to 1cm above the sill.
For below sill length, measure to 15cm below the window, or to 2cm above the radiator if one is positioned below the window.
For floor length, measure to 1cm above the floor.
For a bohemiam, relaxed look, you can have longer length curtains which 'puddle' on the floor (works really well with luxury fabrics, like silk).
Slight adjustments can be made to the hanging height of your finished curtains (excluding tab top style), with the positioning of the hooks on the heading tape.