Everyone will have their own way of working, but to get a wall at an accurate offset, I would for instance: select first wall, Ctrl+Shift Move that wall, double-click wall and enter (offset) position manually. However, there are several SH3D interface tools that might be of help.

• Editing / adjusting wall co-ordinates by editing wall (or furniture) properties (double-click).
• Using numeric entries for walls and rooms, see this recent video tutorial: How to design rooms from their inner dimensions. It describes rooms/inner dimensions, but the same principle applies to walls.
• Use numeric entities to create a temporary wall with a length equal to the offset to create an 'anchor' - and delete the temporary wall when second wall is drawn.
• The Advanced Editing Plugin available from the tools/plugin page will let you move (duplicated) wall(s) a specific distance.

But I guess there are all sorts of ways of doing this. I sometimes create a fake level under my drawing level with walls that can serve as 'anchor' points as a wall/room on a higher level will snap to co-ordinates on lower-level walls. The fake level can be hidden (Ctrl+Shift+H) when not in use, and eventually deleted.
ok