How do I show a percentage value in a views_field?
to achieve some calculations I see 2 different solutions.

1/ You create a custom sql query with (db_query) inside a custom module that gonna trigger a view to render, your select clause will make the calculation for you so you will have your result that you are going to display inside a custom field in your view.
You can see the actual query you are executing by using the devel module and then tweak it.

2/ You can use computed_field module but it's a really dangerous thing to integrate PHP inside a field somewhere, because if your php breaks, you will break your complete site, and need to rollback your database. But I don't suggest this solution because it's really risky for your site.

I hope it helps

Cheers