Measure the full width of your aperture along the floor and mark the centre point (CP).
(O) The overlap on the doors depends on the chosen profile and is normally about 35-50mm.
(P) Add the two floor measurements to either side of the centre point on your diagram.
(Q) Using the plumb line, transfer the centre point at the floor level to the ceiling.
(R+S) Measure the width of the ceiling from each side of the centre point to the start of the angled slope.
(T) Measure the height of the wall at each end from the floor to the start of the angled slope.
(U + V) Measure the lengths of the two angled slopes.