Stator,
JAR OPS 1.535 specifies the obstacle clearance requirements for such aircraft. These include, "The assumed gradient from 50 ft screen height to the point of engine failure is equal to the average all engines operating gradient up to the en-route configuration, multiplied by a factor of 0.77"
The engine is asumed to fail at the cloud base, so the gradient below the cloud base must be multiplied by 0.77. This can be done by multiplying the all engines roc by 0.77, or by multiplying the distance to the cloud base by 1/0.77 which is (approximately) 1.3. This is what the author of the CAP 698 has done but he/she has neglected to explain that fact. Unfortunately most of the course notes issued by the FTOs also fail to explain the reason for the (x 1.3) in these calculations.