In your example you said if guino is high to add, you subtracted.
Wouldn't you want to do this math if guino is high.
(8208/20)*20.14=8265
Or
.14/20=.007
.007x100=.7%
8208+.7%=8265 same as above.
Edit:caught mistake same time as you.
So I think the easy way would be your math just make sure to divide vss by smaller of two readings. Correct?