Hi Colonel
You are absolutely correct with the environmental issues associated with fire fighting foam. Infact, it is one of the worst substances if aloud to get in to the water course, but obviously we are talking about large quantities. However, we should avoid where possible. This information I gained from a brief given to me by the environmental agency direct during a training session a few years ago. Why spend money on training foam when fairy does the trick? You are only trying to demonstrate the blanket effect of the foam, not the burn back properties of the foam in question. As for quantities, just a good squeeze in to the extinguisher is fine and it works a treat. Many delegates comment on the 'nice' smell, but it gives you the opportunity to explain the environmental issues associated with the foam!
Hope that this helps?