I think there were 2 main advantages on the bi-lever bike I saw

1/ the levers were much longer, more like 2 or 3 ft, and they joined to the crank over the back of the seat area via links. The effort then, was a lot more slight, and the scope of the levers fully used all the leg muscles. They must have as I cant imagine what its like to pull a 60 tooth sprocket.

2/ the rider was almost prone, almost head first, arms way out over the headstem to extended handlebars. I think this gave some aerodynamic advantage much like those downhill skiers over jumps.

Anyway that stuff aside, I cant remember what the figure is, but if you are into bikes theres a formula for seat height, possibly 1.09 x inside leg or something. This is to make the leg almost fully extend so that you had to use your ankles as well. If youve done it right you would have considered that.