Google sketchup help required

[TechSupp]

Just started using the wonderful Google Sketchup (how I missed this I'll never know), as we want to look at a few changes to the ICT suite. So after drawing it up I cant seem to figure out how to draw a line that starts say half way up the wall without drawing a line from the base first or moving things relative to a point. Quite experienced at drawing with autocad but not sussed this bit. With autocad I just give it @coords from the last point but cant seem to find the equivalent in Sketchup. But for a free product its amazing so far.