Hi,
I noticed that Jack is no longer online so thought I would offer a comment.
I assume you are trying to change the wall that shows green in the image. I don't know if you realise, but you can just select the green or red end 'button' of the wall and stretch or shrink it. Shrinking it will detatch it from the other wall of course. Using a similar method to what Jack mentioned (with the Commander bar), you can right click, select Lengthen, click on the end of the wall you want to lengthen, then type in the dimension you want to lengthen it by in the Commander bar.
Actually I can't quite follow what you are trying to do. If you change the length of the wall, would you not also want to move the wall attached to it or it will not be attached and won't be a room or would stick out beyond the end wall if you lengthened it?
If you just want to move the end wall in by a specific amount, then just click on the end wall and use the keyboard arrow keys to move it right/ left/ up/ down by whatever you have the Nudge setting set to. Set it to 1/16th inch for example and it will move in that increment for each keyboard press. You can just watch the onscreen dimension until it reaches what you want. (Nudge distance is set in Settings/ Programme Settings/ Drawing Aids). This may help since you said you can't get it exact when drawing the wall in the first place. Personally I don't worry about getting them right when I draw them at first. It is easier to do it very rough, then just drag them to the approximate dimension and then use the arrows to get them exact. It is much quicker.
Welcome to the forum by the way.
Allan