what word spacing?

    I am reading this tutorial here on typography: http://webtypography.net/Rhythm_and_Proportion/Horizontal_Motion/2.1.1 /http://

In chapter 2.1.1 makes a reference to word spacing, it mentions some general guidelines.

The problem is that it stays to them, it does not go any further.

It says for example:

ideal word space varies from one circumstance to another, depending on factors such as letterfit, type color, and size.

Yes, but what type color, letterfit and size.

For example, the typography on my site is blue, 12px, and the font is Berlin Sans FB.

What is the ideal word spacing given the above details?

Thanks.