only pressing enter is of course a little too much of a wish but i am sure i saw a ready grasshopper definition somewhere. here at least a quick tutorial, the offset part comes at 3.17 it offsets in both direction, by pluging in a slider you can choose the value in WYSIWYG manner. if you need it rounded than i am not sure where to find that but maybe thats a start.
hmm maybe here
loads of tutorials on youtube, vimeo and on http://grasshopper3d.com