Well, this is a real pickle because depending on what browse you use, page "delimeters" can be different.
It all depends on the resolution the browser displays in.
What I would do is find out what browse the client is running, and download that browser. Then create a simple HTML template with a table. Maximize the table until is perfectly fits in one page. Then note this width and height of the table and print all your contents in tables (without the border so they don't know it's a table).
Also, resolutions within browsers may not be the same on different operating systems.
Also, if formatting is really a BIG issue, then considering using pdflib or cpdf to generate PDF reports. THis way you can eliminate the worries of finding out perfect resolutions. But then you'll have to experiment with PDF and learn all the functions to generate your reports.
if you reports contain only simple flowing text and tables, then using PDF wouldn't be too much of a pain... but if you get into advanced formatting and images, etc, then you may find yourself having to learn and experiment a lot.
-sridhar