Hi Jeff,

Any value between 22 to 50 pF can be used.

In the old days you would put a fixed cap in the range of 22-50 pF and a variable cap with a range of 22-100 pF. You would adjust the oscillator frequency by turning the variable cap while monitoring some sort of device such as a frequency counter hooked up the output of the oscillator. After you made the adjustment you would usually coat the variable with a dab of special paint.

Most of today’s circuits don't use the variable cap because of the expense. As a result most crystal oscillators are not calibrated at all and they run fast or slow.

Most clock on today's laptops and desktops run about 2 minutes fast or slow a month because of this cheap method of manafacturing.

Regards,

Charles Bannister