For those that might be interested I've found that the following settings in Calibres XXX file to epub conversion give a good approximation to a normal paperback page to my eyes, increase the font numbers by 1 or more if you want the text size larger.
This allows you to directly convert to epub without having to rename/delete the css file and loose other formatting.
Note: Only tested using Hal -Spacejock from it's RTF source file -> ePub. Will try a few more later.
- In Calibre, after adding the book, right click and select Convert eBooks -> Convert Individually.
- Under "Look and Feel" use 14,16,18,20,22,22,24,24 in the font size key, put a tick in the "remove spacing between paragraphs" and drop the text justification box down to "justify"
- Under "Page Setup" change the values from 5.0pts to 3.0pts for each of the margins
This gives a good size for me, but might still be too small for some. Scaling still won't work nativly but at least formatting is retained whilst still allowing a bit of control over font size per book.
Personally I also forced a manual front cover during the conversion, the values above might need tweaking book to book.