Sorry you are having trouble with the margins, but its an easy fix! In the print dialog box, there is an option for "Shrink to Fit Page" or "Scale to Fit" or just "Scale."
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Make sure it is set NOT to scale and it prints at 100%.
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Screen Shots of what you might see...

Safari Browser

Chrome Browser

Step 1: Select Print using System Dialog

Step 2: Click the Preferences button
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Step 3: Choose the Effects Tab, then make sure Resizing Options use "Actual Size."

Firefox

Step 1: You may need to click "Show Details"

Step 2: Uncheck Scaling