A fence, 5m total length.
Post: 0.1mFence: 4.90m
Post sits on 0 0 0, fence segment is 4.9m long. Pivot of the fence segment is at 0 0 0, the segment starts at 0.05 m and ends at 4.95 m.
This means you just have to set the post to 5 and insert it into the fence and the segment ends flush with the post.
Alternatively to nesting, always 5m from the position of the last post.