We quit importing the salt last year, freight costs from Germany and too dealers simply became prohibitive. It is no longer available. The closest salt currently available is hW Reefer which is what our salt was based on, made by the same company Hans Weigandts in Germany. Hawaiian Marine in Houston is the distributor. 713-680-2227.
The partial batch issue exists with all salts to some extent, especially with bulk buckets as the larger grain salts go to the top, smaller grains to the bottom due to settling. The bags are a bit less prone to this as they are very tightly packed and the salt cannot move. The oldest aquarium books always said to mix the entire package at once for this reason.
I don't have the exact figure but roughly 58 gallons.