Looking at specific gravity values for tree species in various publications, to say the least, can be very confusing. For the average person to actually measure it correctly is out of the question. Using a formula, unless you know the exact moisture content (mc), shrinkage (s) and the the fiber saturation point (fsp) of the species can be a guessing game.
To keep it simple, the following calculators all use preset values for fsp, s, and mc values. The result will give you a close average value you are calculating for.
Basic Specific Gravity = ovendry mass/green volume
For all the other specific gravity values you have to read and understand what it says.