I'm using the Pool Calc app on my iPod and in the setup section it asks me to specify the acid strength. It defaults to 31.34%. Is this typical for pool store acid in CA? The bottles I bought from Leslie's don't give any strength information on the box or the label. The other choices are 15.7%, 28.3% and 34.6%. Can anyone confirm what strength acid I'm using or how I can determine it?