The Sori represents the curvature of the Katana blade.
To do this, simply imagine a straight line between the Munemachi and the Kissaki, then from the deepest space between the line and the blade, you draw a line to see the extent of the bend.
Types of curvature
There are several types of bends: bends with the deepest point towards the center of the blade, bends with the deepest point towards the handle and a bend with the deepest point towards the Kissaki.
A more curved blade does not mean that it will cut better, on the contrary. It is important to find the right curvature for a blade that is both strong and not likely to bend and a perfect curvature for cutting.
This curvature is achieved during the differential tempering of the Katana.