I am looking to create a calculated pid for .5V = .68 Lambda and 4.5V = 1.36 Lambda

I understand how to type all the information into the calc_pids file I just am unclear on the math behind coming up with the equation

The equation for the AFR .5V = 10 and 4.5V = 20 is
{EXT.AD1} * 2.5 + 8.75

Can anyone help me understand how to come up with the same for the Lambda values