You're right there is more to it, but it's a reasonable rule of thumb. And of course that's waterline length and width not overall. Lots of long british or greenland style boats have a fairly short waterline length (and low length/width ratio) because of the long overhangs. I should add if you can find a Sea Kayaker magazine review of the boat you want to buy that would ideal as they include speed vs. drag and stability curves for the boats they review.
In addition, lots of race directors use soundrowers boat classification method to determine kayak classes. That's why I included it.