The diameter if the plunger is definitely not the right value to use for slicing, nor is the plastic filament diameter. Use 3.125 mm, which is the internal diameter of the tubing for the Discov3ry.
The unknown factor in your equation is material compressibility. Even if you hardcode all those elements in your equation, the paste material compresses, and this compression coefficient is different per material. Pressure must get built up to overcome this compression and induce flow through the tubing. There is resistance to the flow in the tubing, which is a function of the internal diameter and the length (so don't use a 6-foot section of tubing). The final part in getting the material to print properly is choosing the appropriate nozzle diameter. If too small a nozzle is chosen, the material simply will not extrude fast enough to print, if at all.
Ideal paste materials for 3D printing must exhibit shear thinning. Then the viscosity of the material is the second most important parameter. Both of these parameters together influence the flow of the material, and the steps/mm just becomes a more abstract detail.
Are you building a custom printer and trying to hardcode all these values in?