With respect you are going to need to be a lot more flexible on your % to have any chance of finding someone decent to work with.
You provide the design and that’s it? Presumably you are not offering to support the theme. To update the theme periodically. To bug fix and components or plugins?
The design is only part of the equation - by your offer, you will get between 30-40% of sales for doing nothing now the design is done, while the author has to offer support, updates and so on - no one in their right mind is going to agree to that