Whenever I put in the salt, I never do it by measure...I just put it in, and I always think I have enough...And the sg reads 1.005 and I'm like :WTF: I JUST ADDED LIKE A BUCKET OF SALT.
really don't need a refractometer. For most fish keepers a hydrometer is plenty accurate.
M1V1 = M2V2 - very handy for chemistry class and even here in fixing off salinity. However you can't really start there because of several variables: dissolution rates, temperature etc. However, if you know your concentration and volume for the first mix up, it would be handy for caculating a fix, as all the variable would be set.
Someone has been in their chemistry book. Well done.