Here is a second for you.
The sides of a pyramid are all triangles. You can draw a horizontal line across and it will be proportional to the base based on how far up you are in terms of the height of the triangle. For example, one side of the pyramid is an equilateral triangle with base 230 and height 150.
Let's make X a fraction.
H will be the height of the triangle
W will be the width of the base
For a base Z that is
X*H above the original base, it will have a length of (1-X) * W
A base 50 m(1/3) above the original base will have 2/3(1-X) times the original width of the base.
2/3 * 230 = 153.3~
That is the length of one side of your square cross section.
Hope that made some sense.