I've somehow ruined my coopers hydrometer. When I took the OG of my brew it came out at 1038 which I thought was a little low but I left it. A week later I took another reading and it was down around 1005 which I thought was crazy so I checked it in water and I was 6 points under 1000 . However, if it consistently reads 6 points too low, can I just add 6 to the readings?