I've done the Greensand Way but not exactly sure what you mean by a 'round trip?' If you mean getting to the end and then catching a train home then that's easy, walk to Ashford, go home by rail.
If you mean however, a 'walking round trip,' easily dooable but your choices of return routes to Dorking are endless. I would suggest logging onto Bing Maps, click on their OS section and decide how you might want to do it. There is the High Weald Trail and the Sussex Border Path that can be followed and loads of other paths that can link the two.
En route towards Ashford you will walk through the graveyard of Linton Church, my mother and other members of my family are buried there. A good olde worlde pub opposite the church as well.
Depending on what time of the year you plan to go, a great fruit scrumping route!