UPDATED INTRO
Whereas the simple addition of an alternate stylesheet and extra line in the header, as described below, works in many/most instances, on my site the Print Preview option didn't display as styled by [print.css]

On reading I discovered this was a common problem where the float attribute was used and I had made a styling plugin that used such.

So the purpose of this is to enable a Print Preview option which isn't available using an alternate stylesheet
<link rel="stylesheet" href="print.css" type="text/css" media="print" />

With further thought, following responses from #fredsy I am investigating if it is only a poor implementation of my_plugin that is the problem.
However a side benefit is that I now have the choice of a full=page Print/Print Preview option and Post Only version.

The Modification
There are two steps.
1. Use a copy of [/skins/custom] directory to make a new skin folder [/skins/print].
The new folder should now contain these files:

index.main.php
item.css
_item_block.inc.php
_skin.class.php
style.css

Edit the file [_skin.class.php] in the new [print] directory
Change the class name from “custom_Skin” to “print_Skin” (around line 20).
Then, locate the function get_default_name (around line 28) and change the return value from “Custom” to “print".
Now that line should looks like this return 'print';.

Now you have a working skin. You can check it out by installing and changing the default skin for your blog in the backoffice to Print".
Your blog, it should look like the custom skin now.

You will no doubt modify the [index.main.php] to removed headers, footers and sidebar code etc. that you do not want etc.
I copied ,edited and inserted code from [_html_header.inc.php] and [_item_block.inc.php] into the index.main.php rather than calling them.

[style.css] should be edited to format the page so that the printer friendly page represents as close as possible the Print Preview of 'the' browser.

2. The second part was to edit [/skins/ my_usual_skin/single.main.php] by adding this code just before the edit lnk:
echo'<a href="http://'.$_SERVER['HTTP_HOST'].$_SERVER['REQUEST_URI'].'?tempskin=print&amp;disp='.$disp.'" target="_blank"> Print </a>

It seems Firefox Print Preview will only read from a stylesheet if it's tagged as media="print"
So have two stylesheets, [style.css] for the 'Printer Friendly Page' and [print.css] for the Print Preview.

[/skins/print/style.css] has @import url (../my usual skin/style.css) and body width around 1000px and large margins so that the 'printer friendly page' is similar to the wide margined typical browser print preview. you can see it working at http://calstock.org.uk/elf.php/2014/06/15/ragwort by using the Print link.

[/skins/print/print.css ] has @import url (../my usual skin/style.css) and body width around 1000px, and small margins. I added styling from a plugin I had made to invoke css float: For some reason although the [/skins/print/index.main.php] had php include_headlines() it only worked with Printer Friendly page not the Print preview option in the browser.

3. [/skins/print/index.main.php] just before </body> tag Added javascript alert
<script type="text/javascript">
alert("The layout, especially margins, may be adjusted by your browser.\r\n\r\nYou can check the Print Preview before printing and modify the layout via\r\n\your browser\'s [Print] > [Page Setup] and > [Options]");
</script>