The most expensive curtain mistake is a drop that is almost right. We measure with the track or pole decided, not before. Hardware changes height.
Fullness is a taste decision: too little and the fabric looks mean; too much and a small window disappears. For sheers we usually sit fuller than for a dress curtain.
If you have split ACs or awkward pelmet boxes, say so early. The nicest linen in the book will not save a track that cannot open past a compressor.