Need help with a formula I suck at math

Ok here is the dilema I have a movie clip I want centered its 450px wide and my stage width is variable the part where I’m running into the problem is this… I want to not count the first 300 pixels to the left of the stage

So here is the diagram I came up with to demonstrate

<------------------------------------> 800px stage width
<-------> 300px I want to ignore
<----------------> 450px Clip I want centered on left
over space registration
point is top left

Please help me out guys

EDIT ok that screwed up my formatting on that I’ll post ou a pic shortly