One of my collies regularly eats chocolate and is absolutely fine. Another one stole a 100g bar of plain chocolate, thankfully threw up most of it, but was hyperactive on and off for over a week.
It is this hyperactivity which is the main symptom, due to increased heart rate and metabolism. Also the active ingredient has a half life of, I think it was about 17 hours. That is to say, the ingredient is reabsorbed from the lower gut after it has already been in the bloodstream at a rate of about 50%, so just when you think your dog is calming down they have another burst of hyperactivity. During any one of these hyperactive periods, despite the lower levels of chocolate in the system, their heart can reach the point of overload (having already been stressed by the previous dose). This is why vets treat with activated charcoal, which absorbs the ingredient in the lower gut to reduce further the amount that returns to the blood stream.
Pepper was a very lucky puppy (she was only 4 months old, so not very big). The dose she had would have been fatal in many cases, and would have killed her too if she had not been sick. I have been extremely careful about plain chocolate since then (I had been before, but obviously not careful enough). Milk chocolate is less of a problem, and white chocolate basically contains only trace amounts of the problem compound. Cocoa powder is pretty much lethal in even small amounts, so if even a dusting floats to the floor while baking I immediately clear it up.
Some dogs are more sensitive than others, which is not necessarily to do with weight, either. So the guidance on the website above, which appears to be based on weight alone (although it wouldn't run properly on my pc so I could be wrong), is just that, a guideline.
Bottom line, if your dog has had chocolate and is bouncing off the walls, best get to the vet because a heart attack could occur at any moment.