Since no one has responded, I'll try to give some help (and perspective).
First let me say that designing print layouts and printing them is one of the most frustrating experiences in FileMaker -- so it's not you. It's the same for all of us. FileMaker's roots are, of course, in "data processing reports". If you research the topic sufficiently, you'll find reminders that "FileMaker is not a word processor".
After three years of struggling with the same chopped lines, strange margins and page break "surprises", I can't say I've detected any "hard rules". Rather, it's a case of trial and a lot of errors. I've learned to pay close attention to the following aspects of print layouts:
Be very conscious of the printer-relative "end of page" line across the bottom (in layout mode). If you don't see it, drag the Body line downward until it appears. You must know where your printing will fall relative to the bottom of the page.
This line is adjusted by FileMaker according to the last Print Setup you have performed. If you change printers, FileMaker moves the line but does not adjust the layout accordingly -- it just prints things wrong.
When you place individual fields on the layout, make sure you leave room for descenders -- otherwise, they're clipped. (I have a screen capture file that shows this, but unfortunately, this forum can't accept attachments.)
For fields that can be larger than one page, you must drag the Body end line downward across as many pages as you think the field might ever contain -- although I think the maximum you can specify is about eight. It is my experience that this rather crude method is particularly susceptable to problems when changing to a different printer.
Field Sliding: you can go as near to the bottom as you dare in order to use paper efficiently -- but not too near or the text will be clipped (and you lose the unseen portion).
Regarding footer size, make sure the end-of-page line isn't falling right on the bottom on your layout. It's hard to see, but even harder to find when your printing isn't working. This can easily happen -- and go unnoticed -- if you change printers.
Make the footer as small as practical for your use to avoid wasting printable area on every page.
Those are the main pointers I can think of at the moment. Maybe others can add to the discussion.
I hope (and have since v8.0) that FileMaker will give the entire area of print formatting the much-needed upgrade the database deserves. Otherwise, it will remain a blemish -- and a huge time waster -- on what is otherwise a fine package.