Determine the correct capacitors value

what the right way to determine the correct capacitors value needed for a avr-chip with a crystal oscillator?

Let say the crystal's load capacitance is: 20pF

and i want to use: atmega8-16pu

what should i look for and what the math behind it?

please don't just tell me to use 2x22pF capacitor. I'm not here to do my homework, but to

understand it for future use.

From AVR042, "AVR Hardware Design Considerations", section 4, "Using crystal and ceramic resonators", subsection 2, "General about crystals and ceramic resonators":

$C = 2 ⋅ C_L − C_S$

(CS can be estimated to be 5pF - 10pF)