What will the company lose if you walk? That is what your equity should be based on. Given your brief bio, it would seem as though you can offer a lot to the company.
What contribution are you making to the product - aside from doing 40% of the work, are you contributing to design/product decisions, novel ways to implement the features? Is there any IP onto which your name would be should it be granted a patent?
What other options do you have with respect to other opportunities?
EDIT:
What will the company lose if you walk? That is what your equity should be based on. Given your brief bio, it would seem as though you can offer a lot to the company.
What contribution are you making to the product - aside from doing 40% of the work, are you contributing to design/product decisions, novel ways to implement the features? Is there any IP onto which your name would be should it be granted a patent?
What other options do you have with respect to other opportunities?