Well, to chime in here, I ran your stabilator in XFLR5 to see what it had to say about the situation. It looks as though 20.5cm is just slightly on the unstable side of things. The calculated center of pressure for this particular geometry is 20.2 cm aft of the root leading edge.
The simulation parameters are as follows:
Geometry is consistent with the above dimensions
Fully viscous solution
NACA 0010 foil
Vinf = 100 mph (rough guess, faster is better
this corresponds to a root RE of 1,370,923 and a tip RE of 521,547
Anyways, the attached graphs show a negative pitching moment coefficient (stable) for pivot positions below 20.2cm. Any further aft and you are behind the CP.