Quality Factor of Spiral Inductor for WPT

I'm designing a wireless power transfer system in HFSS using the magnetic near-field. The transmitter can be of any type but the receiver has to be small enough to be integrated within a device (between 1mm to 3mm thick). I'm designing the transmitter to be a helical coil as it has a large quality factor but a spiral, planar coil for the receiver with an external capacitor to match the resonant frequency . I know for the helical coil the length to diameter ratio has to be between 0.5 to 2 for optimal q factor but I can't find any q factor design guidelines for the spiral planar coil. Anyone know how to achive optimal q factor for this coil? Any help would be appreciated! Thanks.

Anyone?? This is really important!