Gravity fed is fine but you can't pull from the very bottom of the container. At least a couple inches up from the bottom is where you want to be, because the slurry will settle at the bottom. You only want to dose saturated lime water and you do not want the slurry getting into the tank.
Most people start off with 1 tsp per gallon of ATO water. 2 tsp is the max that the water can hold. Start with 1 and test daily until you see what your parameters are doing. You might need more, you might need less. As your coral grow, you will need to increase the amount until you max out at the 2 tsp/gallon at which point you might need to supplement with two-part.