The ideal necklace length depends on the individual's build and personal preference, and it might be dictated by the type of preferred neckline.
Here are important considerations:
Where will the bottom of the necklace sit in relation to the preferred neckline?
Does the design include a pendant/s - where will this sit in the cleavage?
Face your client towards a mirror and stand behind her. Drape a long chain, or string, around her neck, holding one end at the back of the neck (or the cleavage point) where the closure would be, and allow the length of chain/string to 'drop' until it reaches the ideal point. Hold the ends tightly as you remove it and measure. in cm and mm.
Ensure that the wrist measurement is not tight. It must be a slightly loose fit as this will be the inner circumference of the end piece. Measure in cm and mm.