In Europe, forklifts rated <5000kg @ 600mm have to be able to lift the rated load on a gradient of 4% up to 3.25m without tipping. ANSI states that the forklift be rated to whatever height it can lift the base capacity to when on a gradient also. If the operator lifts a load until the back wheels lift off, they are lifting way too much & should be aware of why they shouldn't do so. Besides the tyres, forks, fork carriage & mast might not be rated to handle such a load.

Regarding the post above asking if a forklift can lift more @ a smaller load centre than its rated at? Probably not. in most cases the mast will only have a lifting force equal to the rated capacity. ie a forklift rated to lift 4000kg @ 600mm LC can only lift 4000kg @ 100mm LC.
You could however increase the lifting pressure to override this but it wouldn't be recommended at all.

I think the simplest method for an operator to rate the forklift would be to use the linear rating formula (weight x Load Centre / New Load Centre). It's simple, should cause less confusion & its a bit more conservative than the moments formula that uses the distance from the fork face to the drive axle.