Thanks for your help Dan!
It seems to be an easy thing, in theory, to do. Just assign a location (layer)to the focus distance and where ever that layer is in 3d space, it’s always in focus. I must be missing something, usually that is the case with me!! I thought that the apeture value would determine the amount(distance) on either side of the target layer that would be in focus too with a falloff at the edges?