Three lines in one, passing through the midpoint of the bottom edge (that is, the vertical height of the bottom edge) is the vertical line of the waist.

Get a right triangle, the hypotenuse is 3 cm, and the angle of the three lines is 60 degrees.

Then the trilateral ratio is 1k: 2k: √ 3k.

2k = 3cm

K = 1.5 cm

Distance from the midpoint of the bottom edge to the waist √3k= 1.5√3cm.