How draw a circle with spline component?

I create a BP and add a spline component, then in construction script add 4 spline point and set tangent parameter.
The question is How long the tangent is. So I find some document about Bézier curve.The answer is 0.55191502449*R(Your circle’s Radius)

According to the picture up,tangent Length = 0.55191502449R2.But in fact it’s 0.55191502449R3.Why?Why is 3???
This is my question.

I am trying to achieve a circular spline like this one you have made. Could you possibly show me how you did this in your blueprint? I would really appreciate it.