Listen to Corri, Balto and Nop. Don't go down the road of playing games. It is not strong.
Also I spoke with my XH who is an attorney, and a good one, and he said that typically the person who moves out does not pay the mortgage unless they are the one who overwhelmingly makes more money. That said, he understood your point and your strategy. He had also seen situations where the couple put the mortgage and rent into a pot and split that equally. This is just my opinion but I am trying to give you a position where you can stand firm.
After following NOPs script, if you get to talking about finances you can say that "I received a recommendation for you to pay half the mortgage and your full rent. While naturally that is tempting for me because I do not want you to move out, I also recognize that puts an extra burden on you. So I have decided that we can put the mortgage and rent together and split those equally. Then both of us are sharing this extra burden." Note that I would stress that you have made the decision for the two of you.
Just my thoughts...
But what is happiness except the simple harmony between a man and the life he leads? ~Albert Camus
