First draw the shape:

1: Hit Clear + Start and enter first Side Length, then hit appropriate Direction Arrow to draw. Repeat to complete closed slab shape.
Or: Un-check Clear Each Entry and enter a smaller standard Increment Length (1000 ?) then keep tapping Direction Arrows to draw in increments. Hit Undo Last to Undo last drawn side.

2: When shape is complete, it will fill with solid color.
Enter Slab Thick, edge and internal beam thickenings if required, then click Calculate for results.

Starting at top left of plan, enter side lengths clockwise around slab (top left to right) and hit correct direction arrow to draw sides.
The diagram will re-size and re-scale to fit the page as it grows.

Left click and drag to measure in diagram at full scale.

You can also draw the slab shape with Keyboard Cursor Keys.
Hit Clear + Start then hold down the SHIFT key and use cursor keys to draw in each direction.
Un-check Clear Each Entry and enter the Length of each Increment drawn. Use Shift + C to close diagram (when sides align) or Shift + Z to undo last drawn side.