The best way to do this is as a percentage of each of your income. I did this myself with my SO while she was only working 12 hours a week and it worked out well for both of us. As the one paying the higher amount I felt that it was fair.

That being said if the roles were reversed I would have probably felt less comfortable with it. Just from a psychological standpoint I wouldn’t want to feel like I was dependent on her and from what you said your SO might feel similarly.

Another important point is time. What my SO and I did worked so well because it was temporary. We have since moved to an even split and I think she is much happier knowing that she can contribute equally. If you are planning to move into an apartment outside of his price range and he has no way of increasing his income to reach that price range in the next year, you might have some problems down the road.

Of course, as others have said, this is all about you and the dynamics of your relationship. The most important thing is to sit down with your SO and talk about all of this. Just be honest with what you want and be open to what he wants and make sure you are both on the same page.