Rounding numbers in Sass and adjusting the amount of decimals

From the SASS change logs:

The numeric precision of numbers in Sass can now be set using the --precision option to the command line. Additionally, the default number of digits of precision in Sass output can now be changed by setting Sass::Script::Number.precision to an integer (defaults to 3). Since this value can now be changed, the PRECISION constant in Sass::Script::Number has been deprecated. In the unlikely event that you were using it in your code, you should now use Sass::Script::Number.precision_factor instead.

This was added in SASS 3.1.8.

A quick option without any extra functions would be to multiply the number by 1000, then round it, then divide by 1000.

round($percent * 1000) / 1000;