For bleach and chlorinating liquid, you want to use the "Trade %" since that is the volume % of available chlorine. One gallon in 10,000 gallons produces an FC level equal to the Trade % (one liter in 10,000 liters does the same thing). This is because "percent" is per 100 so putting this into one in 10,000 is a net of one in 100*10,000 = 1,000,000 or one in a million which is "parts per million". 6% bleach has a trade % of 6.17 so that means that one gallon in 10,000 gallons will produce 6.17 ppm FC. So (10000/27038)*6.17 * (70/128) = 1.25 ppm FC. You can see that you scale down due to your larger pool (i.e. the same amount of chlorine results in less of a rise in FC since it's in a larger volume of water) and you scale to units of gallons using 128 ounces per gallon.

Then, 1.25 / 4.2 = 29.8%

For practical purposes, you can just use the % Weight of sodium hypochlorite, or 6%, in the calculation and not be that far off. You would get 1.21 ppm FC if you did that.

Richard