Width - Measure track or pole length.
Drop - Measure top of track or pole down to desired finished length of curtain. This could be anything between just below the sill to floor length.
Width - Measure inside window recess across exact (mm).
Drop - Measure from very top of recess to sill exact (mm). Please measure in 3 places and use smallest measurement.
Do not worry about knocking any allowance off your sizes as this will be taken into account when your blind is being made.
For outside recess blind - Simply measure where you would like the blind to sit usually a few inches either side of recess.
Width - Measure inside window recess across. Double this measurement for gather.
Drop - Measure from very top of recess to sill.