The Set Custom Scale dialog lets you set a custom scale for your drawing. You can choose between Imperial and Metric measurements like inches, centimeters, meters, millimeters, and feet and set what distance in real life equals what measurement on the screen.
Default ruler placement:
If you move the rulers by 1 unit, you'll get:
Moving the placement of 0,0 allows you to place shapes and walls at 0,0 and still have room to display dimensions and other information.