The amount of down thrust is really a function of nacelle location. My BE1900 has nacelles like a King Air or anything else with the P&W PT6. With this nacelle the crank shaft is slightly above the wing and the engines thrust will cause a downward moment about the CG. In this situation, no downthrust is needed. Now I understand that you are building a duelist. If I remember correctly, I believe that the crank is just above the leading edge, so I would think that no downthrust would be necessary.