Measuring Self-Capacitance

I would like to determine the self capacitance of a single electrode on a PCB. I have successfully built the model and solved for the capacitance, but I'm not sure of my answer. The process I used is as follows:

1. Create 3D AC/DC->Electrostatics model
2. Use rectangles to create PCB and electrode
3. Create two bounding spheres (r=0.2mm and r=0.3mm) centered at 0,0,0 that encompass the PCB/electrode
4. Select both spheres and create a union
5. Select the sphere union and electrode and take the difference
6. Set the relative permittivity of the PCB (4.8)
7. Set the sphere union as an infinite boundary (spherical)
8. Edit the boundaries so that the infinite boundary is ground and the electrode is port with forced voltage
9. Mesh and refine once
10. Solve and use point analysis to determine capacitance

With a 3mmx3mm pad, I am getting a self capacitance of about 172fF, which makes sense. However, when I increase the spherical boundary size to double check my answer I am getting a different value. Is this simply due to the mesh size changing, or am I doing something wrong?

Is this the best way to measure self capacitance of a single electrode? Also, any tips for optimizing the mesh in this scenario? The computer I'm using can only handle one refinement after initializing the mesh before it runs out of memory.

Thanks,
Chris