fertilator miscalculating?

I just wanted to let you know that the fertilator is not working prpperly for potassium chloride. I entered in .5 tsp. to 40 gallons of water and the reading I got was 1.64 ppm. According to Chuck Gadd's calculator, the same ammount added to 40 gallons results in 10.38 ppm. I thought you should know. The fertilator has been an invaluable tool to me and I think you guys did a great job with it. Thanks,
John