1. Always read the lower meniscus
2. Make sure that the bubbles are blown away. Froth and bubbles make reading difficult. Also suspended CO2 will make the liquid appear to have lower density than reality.
3. Make sure your tall glass or measurement jar or graduated cylinder is long enough (30cm should be fine) to stay afloat.
If you don't have a graduated scale, take a water bottle or pipe (one end sealed/pinched) and make your frugal tall glass.
4. Sometimes the paper scale inside hydrometer slips. However it is a linear scale, so dipping it in RO water will help you compute your error quotient. All readings need to be compensated with that error quotient.
A triple scale has three readings:
1. Specific gravity
3. Potential alcohol %
If your simple hydrometer does not have the triple scale, then download the brix table from Arishtam.com (hydrometer product page) and compute the alcohol and sugar levels.
Now most important part: your alcohol level is the total change in hydrometer reading (initial S.G. - Final S.G) + Delta SG during each step feeding stage. This is the total amount of sugar converted to alcohol/vinegar/lactic acid.