To determine your back profile, lean up against a wall with your heels, butt, shoulders, and head touching that wall. Then, reach back with a closed fist to see if you can easily pass that fist in between the small of your back and the wall. If you can't, or if your back gets pushed away from the wall at all, then choose the "Semi-Flat" back profile option when choosing your frame. This is 95% of people out there, but if you can easily pass your fist through that space, then choose the curved option.

Please watch the following video: 

https://www.instagram.com/tv/B_kS6sSlg-5/

https://www.youtube.com/watch?v=8LuV4CfhvUs