The orientation factor (k²) is 2/3 in quickly reorientating media (e.g. free moleculaes in a low-viscosity solution), but may be between 0 and 4, depending of conditions. The donor quantum yield (φ) is a scaling factor.
The overlap function is a value derived from the integration of spectra of the donor and acceptor, and ranges approximately between 10^8 and 10^16 (consider the 4th power of wavelength in nm multiplied by the molar absorption coefficient).