You don't choose $$\C_{stray}\$$.
$$\C_{stray}\$$ represents parasitic capacitance, mainly between the signal traces connecting the crystal to the rest of the circuit and ground (or whatever other low-impedance nodes are nearby).
The trick is, you generally don't know $$\C_{stray}\$$ very accurately. You have to estimate it from your trace lengths and geometry, and possibly adjust your added capacitors based on measurements after building your first prototype boards.