Although the slopes are marked "obstacle height" they are actually your height above the surface. At the left end you are at zero feet, so the aircraft has just lifted-off the ground. The distance to this point is the take-off run (TOR). At the right end of the slopes you are at 50 ft screen height. The distance to this point is the take-off distance (TOD).
Remember that the various factors that you have just to defactorise the TORA, TODA and ASDA, all came from the take-off distance (TOD). So when you use one of them to enter the graph to find the field limited take-off mass, you are at the end of the TOD (at screen height). This means that you must go down the slopes.
It is unfortunate that the quality of the CAP is too poor to see many of the examples given.