Mark's example is a perfectly usable grid, but that article poses some issues for the uninitiated.

First, there is no indication of how EXACTLY the ratios/dimensions were calculated (see comments for others who encountered the same problem).

Second, the article doesn't indicate WHY that particular grid was picked (i.e., the intended use of the grid). A useful grid should have a purpose.

When I start working on a grid -- especially web grids -- I often use the following "Grid Calculator":

http://www.29digital.net/grid/

While I do think that it is good to be cognizant of the mathematical aspects of a grid, a grid can be unique and flexible, and ultimately it is your personal, eyeballing judgement that will be most helpful.

When I am arhitecting a grid, I use the following steps.

1. How wide will columns of body text be? Maximum? Minimum? Exceptions?

2. How many columns per page? Again, max, minimum, exceptions.

3. On a page with 1 pic, 1 header, and max columns of text, what dimensions do I want.

I typically start with what I feel will be the most common layout and work backward from there. The problem with a math-based grid is that sometimes it isn't mindful of all the different layout combinations.

Ideally, you would sketch out a number of different wireframes and THEN figure out exactly what grid you want to use. Remember, the main purpose of a grid is consistency. Good ratios are simply the gravy.

Several magazines have very consistent modular grid usage. Check out GOOD magazine, they have a nice grid that was created by Area17. Most magazines have a formula, from which you can infer the grid they are using.