Hi! I want to add neck circumference is just one element to consider. The trunk of your neck is where you need to look. This is because regardless of neck circumference, everyone's base of neck varies. Some people have skinny necks, but the base of the neck flares out wider, which means you need a longer necklace. Others have a wider neck circumference, but their base of neck does not flare out, so they would need a shorter length.
Think of your neck as a tree trunk, and the trunk is the necklace would lay. Some tree trunks flare out, while others are straight. Also, depending on your flare, it may cause the pendant to pull up a bit, which then if you made it too short, it will end up creating a choker look. So if you find you have to keep pulling your necklace down, it's too short as your neck trunk probably has a flare. I hope all this neck trunk stuff makes sense, but it matters a lot for short pendants.
I would suggest you try it out for a few days. See where it always ends up laying as you move througout the day. Measure where you want it to be, and ask the SA to adjust according. Don't guess on this stuff if you are picky about lengths (I am super picky). I would also measure it from a fixed point, at the bottom of the hollow of your neck.
Good luck and congrats to you!