Normalised View Port Rect scaling question

Hi,

I already posted this on the official unity forum but I thought I'd post here too to see if anyone can help me out.

I currently have a second camera rendering on top of my main camera in the bottom right of the screen for a minimap. It's settings are: X 0.75 Y 0.1 W 0.2 H 0.4. It works fine in the editor and running the exe at 1280x720, but if I drop the resolution the camera is resized in the frame and doesn't look right. How do I make it automatically resize depending on chosen resolution?