According to ThePoolCalculator, adding 1 gallon of 6% bleach should increase your FC by 2.4. But ... the bleach is being consumed in the algae fight. The calculated amount will be your minimum starting, you'll need more. Dose, Mix, Test, repeat 'til you get to where you need to be. As for a limit, don't go too far over your shock level but some overshoot will be ok, it will be consumed.