The Book Block or text block is the rectangle of text on the page. It doesn’t include running heads or folios. Your margins are the distance in from the edge of the page to define the size of the book block.
Okay, you’ve chosen your font, your page size, and you know where on the page your page numbers will be (and if you’ve got running heads.) Your inside margin – the margin next to the spine – needs to be big enough so that your text isn’t falling into that crevice and is easy to read. Your outside margin needs to be large enough for the reader’s thumb, so that it doesn’t cover the text while they’re holding the book.
So … that’s 1 inch for the interior margin, and .75 inches for the outside margin. You will need to increase the interior margin if your story requires a large number of pages (say, more than 400), because the spine will be thicker and will obscure more area.
Your font size comes from this calculation: your lines need to have between 60 to 70 characters (including spaces.) Subtract the interior margin and the outside margin from the page width, and then … play around. Experiment with your chosen font to see how large it needs to be so that you can average 66 characters per line (that’s regarded as the ideal, historically. A little more makes it easier to justify the paragraphs, but too much makes it harder to read, so don’t go for more than 70.) Don’t be afraid of using decimals – we’re going to use the computer to do the math, and it doesn’t mind, really.
— Time Passes —
Okay, you now have your font size. The line spacing is by default 1.2 times the font size in points … but we’re trying to do better than default. Experiment to see what line spacing makes for the best reading experience; you’ll want to go from 1.1 to maybe even 1.4 times the font size. Again, decimals are not going to be an issue. But your paper supply is, as you’ll have to print out the examples to properly evaluate them.
— Time Passes —
That should be the last experiment you need to do. There is one more decision you need to make at this point – what proportion do you want your book block to be? I’ve swiped the classical proportions from architecture – 1.5, 1.618, and 1.666 (2:3, the Golden Ratio [phi], and 3:5) and I stick with them (and they’re a good fit for the standard trim sizes, oddly enough.)
I’ve set up a Margin Calculator Spreadsheet that will take your trim size and line spacing (also known as leading, pronounced “ledding”, in the trade), and produce the right margins for you based on the three proportions for the book block. You can choose differently, if you like – there are purists that prefer absolutely huge margins, and there are good justifications for them. I just prefer not to make books that way; it’s your book, your choice…
The output of my spreadsheet may confuse you a little, though. Printing uses a different set of measures – the inch is divided into 6 picas, which are then divided into 12 points. It’s kind of like feet and inches on a smaller scale. I’ve got the output shown in inches, picas (3 picas and no points are shown as 3p0), points, and millimeters. And that’s because your software may require you to use a specific measure.