Put my weekend thoughts into action and failed. So had a look at the equations again and couldn’t work out why it was about twenty meters out. Then while debugging it I spotted a twenty that was from another calc. Sure enough adding the offset of that one finally gave me the correct results. Then I made the mistake of taking the limits on the axis to form the bounding box and ended up chopping half the bloody thing off. Still, once I’d produced the corners it all finally worked. So that’s about twenty hours on one set of equations.