Blazed, your hannah pen will be very useful in your grow. You also need a pH pen though if you didn't already know that. I grow in an ebb and flow system that is 24gal max reservoir capacity and about 20gal tray so this is very similar to what you are working with.

Like someone said, you want to be able to flood your plants' roots (or where they will be) without the pump pumping more water than your drain hole can drain (otherwise you will have overflow of the worst variety). Loc is dead on, start with 4 hours gap between floods and keep the flood for 10 mins at a time. Increase as the plants get bigger and as you get into flowering, but the first sign of the plants sagging you should cut back on watering periods.

Unfortunately for your first time with the system you might just have to get some sort of measuring method and actually measure out how much H20 you are putting into the system. Be it with 5 gallon water tanks or individual gallons.