seasalt mix preparation

Regardless of where you prepare your water (in tank, in a separate receptacle) the salinity will have to be fine tuned after the initial mix- it hardly ever comes out exactly right. To measure the specific gravity you will need either a hydrometer (cheap and plenty unreliable) or a refractometer (pricier, but right on the money).

I mix my salt into my water in a large dedicated plastic garbage can with a powerhead to keep things circulating. I measure the water that goes in and measure the salt mix but it is always off a bit. After it mixes well I test the SG and adjust it (either by adding more salt mix or more RO water) until the specific gravity is exactly right.

I'm not sure if I understand. Do you mean that you want to mix the salt, and get it to the correct specific gravity, and then add it to the tank? If that's the case, it will be fine, as long as the temperature is the same as the tank.

If you are asking whether you can make a more concentrated mix in the bucket, then add water once it's in the tank, that may cause a problem. Some of the minerals will precipitate and be lost if you make it too concentrated.