Well looking at my manual with a ruler, 1000fpm is pointing at an altitude just over 200 feet different, and 2000fpm is pointing at an altitude 500 feet different. Beyond that, the VSI is unreadable. IF I was designing such an instrument, I would have had it similarly next to the altitude tape as an indicator that when this one is pointing up, that one will go up too. One looks at both together as they are completely interrelated, so the null point of the VSI should point to current altitude. If we are saying it's predictive for 15 seconds time, then this will quickly break down beyond a few thousand fpm that it is strictly an academic design exercise and of no practical value whatsoever.