mixing water is tricky at first. You see in my 55g, sometimes i forget to put ro/di water + buffer to compensate the evaporation...resulting in higher salinity. Water evaporates, salt stays. Ok let's say i wanted a salinity of 1.024 but now my salinity in my tank raised to 1.026..well i have to put less salt than indicated. So if on a 5g bucket i usually put 2.5 cups of salt, i aim for less...like 2 cups. Good calibration is hard to get when you start, but eventually you'll know how it works and we all have to go trough this. Also, when i put less salt mix than usual in my new water in the bucket...the ph is lower than expected because there is less salt...the buffer power is weaker and my ro/di ( or plain ) water ph is around 7. While trying to maintain at 8.3-8.4, i must use a buffer ( mine is Reef Buffer by Seachem ) to achieve it.