The frequency of oscillation in an oscillator circuit depends on the load capacitance across the Xtl.

The load capacitance, in pF, is the nominal value which when connected in parallel with the Xtl will cause the circuit to oscillate at the required frequency.

It is the value used by the Xtl manufacturer during the grinding and polishing process during which the Xtl is being adjusted precisely to the required frequency.

In practice, the oscillator circuit will inevitably provide its own uncertain capacitance across the Xtl which will usually be different from the nominal value.

So what is required in the oscillator circuit is a variable preset capacitor which allows the total capacitance across the Xtl to be set precisely to the nominal value, and the oscillator frequency to be set precisely to the required frequency.

The preset capacitor should have a minimum value such that the total shunt capacitance is less than the nominal value required by the Xtl. When the preset capacitor is set to its maximum value the total capacitance will be greater than the nominal value.

As a guide, if the intrinsic circuit capacitance is 10 pF, and the required nominal value is 20 pF, then the preset capacitor should be adjustable between 1 and 20 pF.

The required osillator resonant frequency will then occur when the preset capacitor is set to about half of its maximum value.

The whole design and adjustment procedure is very non-critical. Just twiddle the preset capacitor until the oscillation frequency is exactly at the required value.