I have an element that I want to increase in
width:calc(150 + 500 / 100%);
<div style="width:calc(150px + (500 / 100%));position:absolute;top:0;left:0;">This one should get bigger as the page gets smaller</div>
Solution is so simple it's mind-boggling. Move the 500px into the first part and subtract the width.
width:calc(650px - 100%);
Gets wider as its parent gets narrower.