Adding convenience class methods:
+ (CGFloat)relativeWidthForHeight:(CGFloat)height basedOnBoundingBox:(CGSize)boundingBox;
+ (CGFloat)relativeHeightForWidth:(CGFloat)width basedOnBoundingBox:(CGSize)boundingBox;
+ (CGSize)relativeSizeForHeight:(CGFloat)height basedOnBoundingBox:(CGSize)boundingBox;
+ (CGSize)relativeSizeForWidth:(CGFloat)width basedOnBoundingBox:(CGSize)boundingBox;