I find that the rating of a forklift has to take three aspects into account.
1st: the strength of the forks
2nd: the strength of the mast & finally the counterweight of the forklift itself.

If the forks are rated for 2500kg @ 600mm (the formula for them will be (2500 x 600)/(new load centre).
The mast is rated similarly.
For the amount the forklift can counterweight using moments the formula is the same as some guys have mentioned above.
For example. A forklift rated at 3000kg @ 600mm Load centre with front overhang (dist of front axle to front of fork) of 200mm. Has to lift @ 1000mm Load centre the formula is.
(3000*(600+200))/(1000+200).

In a nutshell what I'm saying is that the most a forklift can lift will be what the forks are rated to and after that you can use the moments formula as long as it's values are less than the forks.