I can’t see the coordinates to help you do this but I’m going to try explaining it to you. in the first example on your sheet you can see 2 of the points of the triangle are 1 above the x axis. to reflect it over the x axis you’d go 1 below the x axis. To reflect over the y axis you do the same thing. Looking at the point (1,1) I think still on the triangle one. You see it’s 1 away from the y axis so you’d do the same thing as in the x. it’d be (-1,1).
IF the estimates are 100% accurate, then the maximum volume of concrete is the product of the maximum dimensions, namely Vmax=18.32*12.22*0.55=123.13 m^3 and the minimum volume of concrete is the product of the minimum dimensions Vmin=18.28*12.18*0.45=100.19 m^3 the uncertainty is therefore 123.13-100.19=22.94 m^3
On the practical side, to this uncertainty must be added to - squareness of the formwork - evenness of the surface of the crushed stones on which the foundation sits on - excess of concrete delivered by the truck - volume of air bubbles trapped in the the concrete, ...