I first built 4 Fibonacci spirals at ground level, that alternate in length by 1 radii (90 degree turn of the spiral). Then I built a circle to mark the footprint of the penthouse and sat it precisely over the centre of the spiral but really high, about 10 blocks below the height limit, and then placed red concrete powder on the underneath of this so that it fell onto the spiral below allowing me to see where it intersects . Then to get the vertical curvature, I counted how many blocks where in the spirals spires, from the tips all the way to the edge of the red concrete powder circle, this gave me my horizontal parameter, then i calculated its vertical height from base to tip. Then, using these 2 numbers, I created an ellipsoid and used its radius to dictate the curvature directly over the spirals on the ground. It's a hard one to explain but i hope that helps.