Almost zero.
Very, very, very few players get "turned into" capable key forwards - it's the position on the ground that is most reliant on natural ability.
Shannon Neale is currently in that process, though he played as a ruck/forward as a junior. We did it with Harry Taylor and Ben Graham, though neither of them really looked natural as key forwards with the exception of Graham's final season at the Cattery in 2004.
It's one of the reasons why two-ruck setups have almost died out - because the old era of resting a ruckman in the forward pocket has gone the way of the dodo. Probably David Hale was the best at it from the modern era.
Hence why Jackson is worth the money Fremantle spent on him.
A capable two way bigman yet to hit his prime is what we all hope Neale or De Koning can become.