Since no one has really answered this question, I thought I would.
FreeSans does a good job of mimicking the default MS Word settings (Calibri 11) at these settings:
Paragraph/Spacing/Below Paragraph: 0.12"
Paragraph/Line Spacing/Proportional of: 125%
Although not perfect, the average bear will not be able to tell the difference without doing a critical comparison against another document.
Although not technically as close, I am also impressed with Verdana 9 set at the same spacing as above. I'm not sure why, to my eye it does an even better job of imatiting Calibri. But either one should work.
I came accross this thread by chance...
The free and open source font Asap is very close to Calibri, much more than FreeSans or Open Sans.
It is a tiny bit bigger than Calibri, so just check your line spacing and font sizes.
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