I'm in the process of trying to shock my spa (325 gallons) with liquid chlorine 7.5%. I had been using MPS but get an allergic reaction when using it so I thought I'd try switching to liquid chlorine. I've tried, quite a few times now, adding 6-8 oz of chlorine to get the water up to shock level. I've been waiting about 15-30 minutes or so to test and the highest level of chlorine I'm getting up to is roughly 4 ppm. Based on a chlorine calculator I referenced it should be around 12 ppm with that amount of chlorine. Is there something I'm missing here? I've tried a couple different bottles of chlorine thinking maybe the chlorine was the problem.