No matter how I do the math Doc, I can only come up with 3 Gallons with 10% concentration from a 5LB Potassium Sulfate as you claimed, not over 6 gallons. If it was for 6 gallons, then it should be a concentration of 5% only.
Here's my math:
1 pound = 454 grams
5 pounds = 2270 grams
50% to 10%...