Hello I'm new to the forums and looking for a little help please

I'm trying to figure out a formula for the following.

Lets say I have 400 as the target number.



You get a score of 450, which is 89% for the target of 400.

Now lets say you get a score of 350 ... I'd want that to come out as 100% since it is lower then the target.

hope that makes sense... Thank you !