Thx for the help
I've realised one of my mistakes - to think of 1em as being a set size. I've realised it isn't. I found a good explanation online. Thx.
So my next question is...looking at the 0.75em code - what is the font size? The code only states an em size, so how does sigil (or the ebook reader) know what the font size is? Or is this based on the stock setting under appearances??
Diapdealer - here is the code for the book with 0.75em. I think you're right.
Code:
<body class="calibre">
<div class="calibre1">
<div class="s1">
<p class="calibre2"><span class="none1">the text</span></p>