It is fairly simple to calculate this based on the angle of the offset vector once normalised. However, something that is important is to understand that a node with an extreme size along one axis will still be a problem. In my case, this won't be an issue but for some, it will. An example would be a long pipe that has a narrow radius but extends off down the z-axis by 100 units. If the camera is viewing this looking directly down the z axis the camera would only see the end, but Mersey-Viking's algorithm would position the camera way too far back accounting for the maximum extent i.e. the length.
I have found another article in the Unity docs: https://docs.unity3d.com/Manual/FrustumSizeAtDistance.html This looks to be more accurate when I finish work I am going to have a play with this and see what it produces.
Also, a shout out to Mersey-Viking for the algorithm should he/she ever stray into these here parts.