Do you think I should get something to take as an example Exactly 5 meters apart, put it on the floor and scan it and use it to define the distance?
X----5m----X
Yes basically a big construction ruler on which you will then put the control points directly on the thin lines.
Yes I have measured the side of the house very accurately, I think you think it is too imprecise to define in RC?
Yes indeed it may be off a little and even a slight inaccuracy to real word might mess up the scale a lot.