Thanks, those links are awesome.
I think think perhaps the reason my SG was on the low side may have had something to do with my sample. I took my sample after topping up my brew . . . it probably wasnt a good representative sample . . . probably a bit on the thin side.
Do you stir you brew once you top it up? My instructions didnt tell me to, and I was kind of drunk and paranoid of contamination at that point, so I topped it up, took the SG and sealed it up. Should I have stirred it up?