Okay, then that sucker is incredibly simple. It's a linear equation.

If you want to display as AFR (Gasoline) instead of Lambda (any fuel), then:

AFR:1 = N*(10 / 256) + 10

For example, if N is 128 (~2.5V), then:

AFR:1 = 128*(10/256) + 10
AFR:1 = 128*(0.0390625) + 10
AFR:1 = 5 + 10
AFR:1 = 15
15:1 AFR