I calibrate my refractometer everytime I use it. My refractometer seems to be decalibrated everytime I use it, usually once a week. You shouldn't take a chance on using a decalibrated refractometer, especially when it just takes a minute to calibrate it. Mix your salt and let it sit for 24-48 hrs in a big tub/garbage can before you check the salinity. Make sure you have a powerhead to mix your water in the tub. Good luck in starting your tank Peter.