Here is a link to a post that describe how you can use interface builder and proportional constraints (%): http://stackoverflow.com/a/26323745
If the direct link doesn’t work, make sure to go to the post that start with:
Using Xcode 6.0, you can now specify proportional width or height in Interface Builder. Steps for percentage height from superview:
While both the child view and its superview are selected, add an “equal height” constraint (or “equal width” if you wish to have a proportional width).