Checking Kepler's Third Law

For Earth and the inner planets (you'll need to view this in landscape mode on a phone):

Planet	Orbit radius (r) in AUs	r^3	Period (t) in years	t^2	r^3/t^2
Mercury	~sin 22.5*	~0.06	7/29	0.06	0.96
Venus	~sin 46*	~0.37	8/13	0.38	0.98
Earth	1	1	1	1	1

(*approximate average greatest elongation in degrees, taken from data here)

So far, so good, but to extend this to the outer planets, I'll need a method of determining the orbital radii for those. 🤔