To tackle this, they use two angle gauges and measure in two directions. The angle gauges are connected and you know the angle between them, their mutual angle. Two angle gauges (with plumb lines or bubbles or whatever) are placed on the slab in two directions. The direction of is the direction that the windmill needs to be “leaned” so that it will stand straight. In that last step you know that since the tower, , and any direction on the slab it’s on, like , are perpendicular.