Optimum RC values for crystal oscillator?

I made a low frequency crystal oscillator using a couple CMOS
inverters (CD4069) and a very old 31.5KHz crystal. It's a fairly large
crystal package that measures about 3/4 by 3/8 inch from the 1970s.

The first inverter feeds the second, and the output of the second
connects the crystal back to the input of the first inverter. The
first inverter has a 100K resistor connected from input to output and
there is a 470pF cap from the input to ground. Works well, but I'm
wondering what the best RC values should be? I was thinking the
reactance of the cap should be about equal to the resistor, but that
works out to about 50pF which doesn't work. Lots of different
combinations will work, such as 22K and 1000pF.

How does one determine the best values?

-Bill
.

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