[o0CY0o]

I'm here to ask about the relationship of the radius ratio with Coordination number.
As i know, we could calculate the r/R ratio from the coordination geometry.
Say for trigonal planar structure, we could assume that all anion touches each other. Then r/R is 0.155.
Yet, i want to know why the upper limit for ,say trigonal planar structure, has r/R value of 0.225.

Thanks