Thank you Shane826 for your reply
I DO NOT want any surprises so checking that the valve outside is in the closed position is a MUST.
So in order to get that extra gallon or so that you’ve explained, after releasing the contents into the plumbing, would I have to close the lever that’s at the bottom of the toilet and recharge the toilet with clean water once again or would that take away from the extra gallon of space that you’ve talked about ?