To get the bottom of the lower column bases, extend the rays of octagonal symmetry until they intersect the vertical axes of the columns framing the lateral entries; these intersections fall at height .109.

The tops of the column shafts in the lower zone appear to fall at height .333, or one third, which can be found by intersecting the half-diagonal of the red square with the diagonal of the square’s lower quadrant.

To get the top of the main capitals, begin by subdividing the interval between the two inner major column sets, and then swing an arc tangent to this line until it intersects the diagonal of the red square, which will happen at height .765.

The top of the main entablature falls at height .914, aligned with the corner of a great octagon of height 2.000 concentric with the red square of height 1.000.

The tops of the shafts in the upper wall niches fall at height 1.250, as the diagonals from height 1.000 indicate.

The tops of the lateral pediments seem to have inner tips at height 1.618, i.e. The Golden Section of the original square.

The top of the main central pediment falls at height 1.750, as the diagonals from height 1.500 indicate, and the raking angle of its sides can be found by connecting this vertex to the points at height 1.500 on the entry axes.

Voila…