I don't know what you did exactly in the ".785x.33x.33.320x7.5=205.16" part, but this is what I get.

Volume = \(\displaystyle \pi (\frac{d}{2})^2 h = \pi (4)(320 \times 12) = 48254.86316\ in^3 = 208.895 511 52\ gallon\)

[Using conversion from

here]

Now, you have 10 lbs of 65% NaClO, this means the mass of NaClO present actually is 6.5 lbs.

So, you have 6.5 lbs in 209 gal (3 sf)

In one gallon, you get 6.5/209 = 0.0311 lbs (3 sf)

Converting to ppm, we get 0.0311 lbs = 3730 ppm (3 sf)

Wow, this is very high for drinking water... or even for bleaching... or do you mean something else by "10lbs (65%) hth sodium hyprocloride"?