You're right the 6% at $2.00/gallon and the 12.5% at $4.00/gallon are basically the same price per pound of chlorine. If you're clearing the shelves, there's no loss in clearing the 12.5% as well as the 6%. use the 12.5% first 'cause it won't last as long.

FWIW - my opinion - get rid of the Frog. At best, it will add copper which will cause you a problem later. Just say no to metal(mineral) algae control. It doesn't kill fecal coliform, only algae. We can eaisily see when algae gets out of control, but, butt bacteria you can't see, but, it'll make you really sick.