Calculating composite wire gauge

Can anyone tell me how to calculate the composite wire gauge for a pair of wires of different gauges? Or better still, for any number N of wires of N different gauges? I know how it works for two wires of the SAME gauge. For example, two 22 gauge wires combine to yield a 19 gauge composite: you subtract 3 from the common gauge 22. But how, for example, do a 22 gauge wire and a 19 gauge wire form a composite gauge? I can pin it down this much: two 22's yield 19, as in my first example, and two 19's yield 16, by subtracting 3, so a 22 and a 19 must yield a value between 16 and 19. That's as far as I can take it. Anyone know an equation?