[sdiy] 2164 Expo correction

[David G. Dixon]

Here's an interesting thing: if you assume the gain constant I measured on
V2164 rather than the one in the datasheet, and recalculate the magic tempco
voltage (assuming the gain factor to be constant and taking the value at 0V
on the VC pin), it comes out to 0.2701V rather than 0.2895V.  This is
essentially the same voltage measured by Sam Hoshuyama on his breadboarded
circuit, which might suggest that the SSM2164 also has this "wrong" gain
factor.