$base-font-size guidelines in _settings.scss don't work

So I'm trying to reduce the standard font size from 16px to 14px. So I've adjusted the $base-font-size to 95% which then makes the grid smaller.

The instructions in _settings.scss say to "set the value of $em-base to $base-font-size ($em-base: $base-font-size;)". If I do that, I get a compile error saying:

"Syntax error: 9.25301em*px/% isn't a valid CSS value."

It doesn't seem to be particularly straight forward to adjust the font size and maintain the original grid dimensions.