Where are you getting min & max voltage of .5 & 4.5? Change that to 0 & 5, and use the max AFR of 16 (not 20) as specified on page 3 of the AFX manual. In addition, page 12 of the AFX manual says the formula for lambda is this:
lambda=(Vout * 0.096)+0.62
The numbers in the manual are no doubt truncated for print. Since HPT uses division, use the reciprocal of 0.096 (10.4167) in the formula:
Volts
-------- + 0.62
10.4167
Lambda is preferred, but if you need to use AFR Error for some reason, then use this because the sensor assumes stoich is 14.57 regardless of what you have in your calibration:
(5-0)
------- = 0.71423
(16-9)
So the AFR formula is:
Volts
--------- + 9
0.7143
Mathematically you can get the divisors a little more accurate, but it's probably statistically irrelevant.