Check if two arrays are parallel to each other and horizontalness? (Bollinger Bands)

Is there a way to compare two arrays and get a double between 0 and 1 where 0 would be 90 degree difference between two arrays and 1 would be perfectly parallel? Something like this would be extremely useful for Bollinger Bands and other indicators with similar structure. Also is there a way to check flatness of an array? Couldn't find anything about this subject for some reason. Any help would be appreciated.