To help you draw walls precisely, use the wall tooltip and the alignment lines. If you press the enter key when the plan has the focus, the values displayed in the tooltip will become editable to let you enter with the keyboard the length, the angle and the thickness of the current wall (if you didn't start any wall yet, the tooltip will display the coordinates of the start point of the new wall). Use the up arrow, down arrow or tab keys to edit an other value and press again the enter key to confirm the values you entered. If you want to use again the mouse to draw walls, simply move it over the plan.