Getting a reliable SG should be fairly easy with a stone this size. I use an Ainsworth Analytical Balance with a sensitivity of 1/40 of a milligram (0.000025gm). However, you can get a good reading even with a Ohaus Dial-A Gram scale (has a pedestal to support a beaker, etc). I don’t recommend using string. Suggest using a thin bailing wire. String entrains air and will throw your wet weight measurement off…sometimes very significantly. Also, weigh the stone dry both with and without the wire so you know the wire weight. Then when weighing it in distilled water, go in deep enough to just completely submerge the stone. Most of the wire will be out of the water. Subtracting the wire weight from the wet weight will give a closer representation of the stone’s actual wet weight.