SG is simple school physics, its the weight of the liquid of same volume compared to a standard which is water meaning
1 litre of water = 1000 grams (metric standard) means SG = 1.000
battery electrolyte of 1.270 SG means that 1 litre volume of it should weigh 1270 grams. When I made electrolyte for batteries I checked with my cheapa$$ hydrometer first then a digital scales too - you can ask your freindly parchoon ki dukaan for this or buy a small sized digital scale.