Borrow or buy a refractometer. If someone nearby has one, calibrate your hydrometer with their refracto because you'll probably find that its off by quite a bit. Friend of mine had 3 and all her hydrometers were pieces of cr@p that ended up crashing her tank
Now she's got a refractometer like mine. Works great & worth the investment ($65 at J&L or OA).
JMO & JME,
Anthony