budcoh, this information is not a simple formula, but for the USA, you can find the design criteria about forklifts (that is included in ANSI and OSHA regs by referring to them) in the standards at the web site of the "Industrial Truck Standards Development Foundation" which is "itsdf.com".
To be sure you were within legal standards should an accident or injury happen, the manufacturer of each truck would have to provide you with 'de-rate' information about that truck and it's added attachments.
I have often seen (for the standard 3 to 5k truck) a "quick and dirty" guesstamate of about 100 to 150 pounds per inch away from the face of the forks to the loads center of gravity, as the de-rate.
The load center is calculated from the face of the forks, (both vertical and horizontal equally), so don't bother looking for a center of gravity vertically higher the 24 inches (610 mm)? But keeping the center of gravity of the whole load and attachments within the stability pyramid remains the goal.

To answer Shamus' question, without seeing the plate in question, it is a bit difficult to be positive as to how to teach this info, but as I understand it: "7mtr lift Load centers 500 600 700, (respective of) Capacity 500kg 450 kg and 400 kg" (implies a "respective of), if your load center is 500 centimeters from the face of the forks, you can safely lift 500kg to a height of 7 meter, if your load center is 600 centimeters from the face of the forks, you can safely lift only 450 kg to 7 meters, and if your load's center of gravity is 700 centimeters from the face of the forks, you can only lift 400KG to 7 meters. Of course this would change if an attachment had been added to the standard mast and carriage the unit had been first delivered with.

Shamus: I don't know the answer to your question but have a similar one. Does anyone know either an ANSI or OSHA regulation (guideline) that addresses derating the forklift's capacity when handling loads that have a center of gravity vertically higher the 24 inches (610 mm)?