Why must a round trip appraise the return separately rather than reverse the outward assumptions?
Short answer
Later tide, weather, daylight, traffic, vessel state and crew condition can change the route and its margins
The return happens at another time and may be a different operating problem even on the same charted line. Mirroring the outward assumptions hides later constraints. Revise The Return and Shore Plan for the separate return check.
Quick check
Why must a round trip appraise the return separately rather than reverse the outward assumptions?