Does anyone know the relationship between Hydrometer Accuracy and
Barometric Pressure?

That is, what is the inaccuracy of a Hydrometer with variations in
atmospheric pressure? I assume that accuracy would depend upon total
surface area of liquid exposed to atmospheric pressure whilst taking a
reading, but does anyone know what % of error there is in the reading?

Say you take a reading @20C and atmospheric pressure is say 1000mb and
the SG is 0.995, then what would the hydrometer read @ 1030mb? Lets
assume that the container we are measuring has a surface area of
126cm2.

Cheers,
Steve!