BH, it looks like it's full of holes to me -- but the basic idea makes sense. Seat-of-the-pants purchases can bust you pretty quickly, so you'd better get a handle on how much you can afford.
However, one number that's very wobbly here is "average number of sessions per month". What if a website is both sales and service? All those service oriented sessions don't belong in this calculation. This calculator assumes that the site is pure marketing/sales.
One concept I almost never hear kicked around online is the "lifetime value of a customer". It's really common in the bricks world to look at that number when deciding whether the "cost to acquire a new customer" makes sense. Of course, the web is probably too new to KNOW the lifetime of a customer.
My sense is that the offline world puts a lot more energy into retention marketing than the online world has done so far. That's just one more reason for the dot-com bust of recent days.