This article was the best I could come up with to explain the phosphorus in fertilizers.
https://extension.umn.edu/phos...
It appears the total, the available and the water soluble are dependent on the process used. I could never really get a grip on the differences in how the plant available and the water soluble differed when applied to the soil. I am not even sure if one cup of your 6 percent would even be closely equivalent to two cups of your 3 percent product. Are they both pretty much equal in particle size? Amazing how something that looks so simple on the surface can be so confusing when you start thinking about it. Let us know what you figure out.