If you have both a hydrometer and a refractometer the original gravity can be determined
OGR = -400(FGH) + 2.512(FGR)+400.52
And to convert back to specific gravity
OGH = 0.00432(OGR)+0.9977
as derived from: OGR = 231.61(OGH-0.9977) reference (4)
Putting it all together:
OGH = -1.728(FGH) +...