Printer drivers for Mac and windows don't scale the same way.
You are more likely to get predictable results if you create a second layout has it's layout elements sized to print correctly with no scaling. Since text on a windows machine takes up a bit more space than the same text on a Mac, you generally have fewer cross platform issues if you design the layout on a windows machine instead of the mac.
For printing, you switch to your new layout to print and then return to your original layout for data-entry and browsing purposes.
Thanks Phil - I was afraid that was the case - was holding out hope for a simpler solution which didn't involve going to a different layout.