PIggybacking a Rear Sac

I have a 750lb sac that I want to piggyback on to my factory centre tank. I have all the necessary fittings , hose etc. so I don't wanna get a wakemakers kit for this one. I understand the fill and vent. But how do I get it to drain and where should I connect the drain line to. I am assuming it will gravity drain back to the centre tank if I hook it up to it. Right now there is a aerator pump attached to the factory tank for draining. Should I take it off put a T in there then run the drain hose to it? or is there a better spot?