I think Fitzhugh is asking something slightly different (apologies if wrong). I have two readers which have very different needs. My Cool-er does not allow modification to margins, line height and best suits my needs with a specific base font. These need to be defined in the CSS of the book concerned. I also have to embed my favoured font in the book in order to use it on the Cool-er.
My Kobo Touch allows all but the base font to be changed on the reader UNLESS they are predefined in the book's CSS.
The above means I need very different conversions for the books I wish to put on each ereader.
I also do ebook conversions for non PC literate friends who have ereaders with other characteristics, which means I use regularly use other settings.
This means I may wish to convert a book several times, over a period of time, each conversion having a set of parameters to suit the ereader it will be used on.
I read Fithugh as being in a similar situation.
The way I tackle the proble is given in the following:-
https://www.mobileread.com/forums/sho...d.php?t=160581
If anyone has a better way I would appreciate knowing what it is.