Put one of your long garments on the usual type of hanger you use, hold it up against the wall so that there is floor clearance and measure. Add a few centimetres to allow for hanger differences (there seems to be a lot).
I used this method for putting hanging rails in my daughter's walk-in robe. She has mostly double space, plus a full length bit at the end. Worked fine - until she bought a dress with a bit of a trainy thing at the back!!
What I didn't allow for was longer length (mid-calf) skirts - they have to go with the long dresses. Next time, I'll measure EVERYTHING.