Not certain what you mean by the carbon foot print. Are you referring to the in plant air quality where these 40 forklifts are operating? You need to clarify this first. But my initial thoughts you might find the approach you wish to take very difficult & some what unreliable as you would need to know the per cent of time each forklift is operated at idle , high speed no load & under max load. - but in reality forklift engine - generally operate a levels below these points (except at idle).

The data provided by Hyster is in a common format used by lift truck companies to demonstrate compliance to various engine emission standard under various accepted & standardized engine running conditions with an engine in top shape - like new. In the US forklift must comply to our EPA, Tier IV standards at the time of manufacture. But the end user needs to be concerned about the air quality in side the building which is more related to the air exchange system than the actual lift truck. This is to say you can have the cleanest burning engine you can find & without adequate air exchange you can exceed in plant emission standards.
There are "sniffers" than can be used to measure you in plant air quality levels.