Your' wellcome
My experience to hold work low is to reformat epubs in Calibre over CONVERTING to epub. Even if the book is an epub I do this 'conversion':
EPUB --> EPUB.
That rewrites the CSS of the book to a more uniform kind.
After you have produced your CSS-files for PRS+ (I have approx. 15 styles covering common combinations of size, fonttype and linespace) it's very quick using them.
A.