Load Capacitors and Crystals
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.