Formula: C_L = (C1×C2)/(C1+C2) + C_stray. For the common case of matched capacitors (C1=C2=C_x), this simplifies to C_L = C_x/2 + C_stray, so C_x = 2×(C_L − C_stray). Using the wrong load capacitance shifts the oscillator's actual frequency slightly off nominal (a real but usually small effect, often only a few tens of ppm) — always match C_L to what's printed on your specific crystal's datasheet, not a generic guess.