@Izzy78 - Like the article mentions, a lot of the leverage created by the Tommy bar depends on the length of the bar. The longer it is, the more force you can apply to the tool. The length of the tool itself would make a difference, too. I would guarantee that there is an equation somewhere, but I doubt it's very commonly used.
My question about Tommy bars and box spanners in general, is why would you choose to use them instead of a normal socket wrench set? It seems to me that with the Tommy bar, you would have to make sure you have extra room to work compared to using a ratchet that can work in a fairly small space. That doesn't even include the space needed to fit your hands to turn the spanner setup.