As a Naval Architect, this is a question that I am often asked. The centre of effort is very close to the geometric centre of the spade as you know.
Is there a fixed skeg or keel immediately in front of the rudder? That affects the hydrodynamics significantly because the hydrofoil would then have an effective length that is the skeg width (aligned with the fluid flow) plus the rudder chord length. While not precise, this approximation will allow a better calculation of Rn (Reynolds Number) to calculate lift.
The rudder torque is the response to the hydrodynamic pressure on the moving part of the hydrofoil. A perfect spade rudder would be long and thin (think aircraft wing), with its axis approximately 1/3 to 1/2 chord from the leading edge at the centre of lift. Unfortunately with ships the rudder tends to be short and wide and much less efficient, so to achieve the same force, a larger profile area is needed ( F= 1/2 PAV^2) P=fluid density, A = Foil profile area, V=fluid velocity.
Adding a skeg achieves exactly the same result as an aircraft wing with ailerons. The ailerons change the lift characteristic of the foil in two ways - firstly the fluid flow is diverted, changing the relationship of the pressure over the top and bottom surface; secondly the centre of lift moves forward slightly as the aerofoil deflection increases, and therefore increasing effort is needed to operate the aileron until stall is achieved. However the actual effort required with a skeg vs. a free spade rudder is significantly lower since the fixed skeg is part of the foil and carries most of the load.
So the answer to your question depends on the physical geometry required, the fluid flow rate, fluid density, and the aerofoil shape itself.
Some of the low speed Selig or NACA series foils may be suitable - eg NACA 0015 with the movable portion of the foil (rudder) at 50-60% chord from the LE of the skeg. Obviously a leading edge radius on the rudder and a trailing edge radius on the skeg are required. The gap between should be minimised.