Hi - I think you want to know how to calculate the required angle of rotation from the linear velocity, right?
If you work in radians that works easier for Godot
1 rotation = 2 * PI radians.
So, to calculate rotation in radians you need
(linear_vel.x / circumference) * 2 * PI
You were ALMOST correct :-)
So, with your figures:
D = 64, so circumference = 64 * PI = 201
if linear_velocity.x = 50, that is
50/201 * 360 = 89.5 degrees
50/201 * w PI = 1.56 radians