If you don't have the specs for the particular electret, choose a resistor (by trial-and-error) that gives you about half the power supply voltage. Or, you could use a pot and adjust to 1/2 voltage.
It shouldn't be super-critical since the signal is very-low relative to the bias.
Don't forget to add a series capacitor to isolate the bias from the signal. And, I assume you know that you'll need a preamp.