When measuring the HEIGHTS of your room (D and C) from floor to ceiling:
If you have carpet, push down hard. Ignore skirting boards or coving and measure from the floor to the ceiling.
Measure in three places - in the centre and to the left and right side and give the lowest measurement.
When measuring the WIDTHS of your room (A and B) from wall-to-wall:
Measure from wall to wall, ignoring any skirting board.
Measure in three places - in the centre and to the top and bottom and give the lowest measurement.