There is no out of the box attribute called "% Schedule". The available ones are listed in the screenshot below.
However, if you are planning to create a custom attribute then you have understand the purpose of the attribute first, like what exactly you want to show there. Based on that you can create a calculated attribute. By the name "% Schedule" I am having a feeling that you are looking for something almost similar to "% Complete", which is already there. Hope this helps.