The following formula may be used to calculate a parallel resonant crystal's external load capacitors:
CL = ((CX1 x CX2) / (CX1 + CX2)) + Cstray
CL = the crystal load capacitance
Cstray = the stray capacitance in the oscillator circuit, which will normally be in the 2pF to 5pF range.
Assuming that CX1=CX2 then the equation becomes:
CL = ((CX1 x CX1) / (2 x CX1)) + Cstray
CL = (CX1 / 2) + Cstray
Rearranging the equation, we can find the external load capacitor value:
CX1 = 2(CL - Cstray)
For example, if the crystal load capacitance is 15pF, and assuming Cstray=2pF, then:
CX1 = CX2 = 2(15pF - 2pF) = 26pF
It is difficult to know exactly what the stray capacitance is, but if you find the oscillation frequency is too high, the load capacitor values can be increased. If the frequency is too low, the load capacitors can be decreased.
The device data sheet may also define a maximum crystal series resistance Rs.