This can all be achieved with a custom formula field (see how to create one here) and you can avoid Process Builder altogether.
Add all these conditions to the formula field, and anytime conditions don't match properly, you can put the value as null
so it'll be blank for situations in which there shouldn't be a rating.
If you need this rating to be static and unchanging at the time it's set (and not always updating with changing conditions), have the same formula field in the background (i.e. take it off any of the page layouts) and use Process Builder to simply set the normal Rating
custom field to the Calculated Rating
custom formula field.