AutoCad :: Select Line Start / End Point That Is Known Distance From OSnap Point?

I'm trying to draw a plan of my house in Autocad 2009 but I seem to be endlessly using trim and extend because I can't easily select the points that I want with osnap. Eg. imagine I have a box, and I want to start drawing another box inside it that has each side as 10 units smaller than the corresponding side on the outer box.

Step 1 - I draw a small line from the top left corner of the big box using the endpoint snap, typing in "5" for the length.

Step 2 - I then do the same at right angles to the first line to get me to the starting point of the inner box. This always leaves me with 2 small lines to remember to delete.

Step 3 - Then in order to draw the top line of the inner box I end up drawing to a random length because I can't say "draw until 5 units away from the right hand side of the big box", then I end up trimming it later once I've repeated steps 1,2 and 3 for the next side of the inner box.

I know in my example above I could have scaled the box down or something but that's not what I'm getting at. I really want a way of selecting line start and end points that are a known position away from an existing point.