How to calculate CL and external capacitance?

The calculation formula is as follows:
C_L=(C_L1XC_L2)/ (C_L1+C_L2)+C_s

Where:
C_L1and C_L2are the external capacitors in the oscillator circuit.
C_sis the stray capacitance in the circuit, generally about 2-5 pF.

Stray capacitance refers to the capacitance that includes:
The capacitance within the IC itself.
The capacitance between the layout traces.
The capacitance between different layers of the PCB, and so on.