Hi there,
Another way of looking at it is the asymptotic approximation for Bode plots is 20db per decade per order.
You also have to check for the break frequencies so you know were the asymptote begins.
For example, a very simple RC filter made up of a resistor of 1592 ohms and cap of 0.1uf has a break frequency of 1kHz, so it is considered 0db at 1kHz even though it isnt and that decreases by 20db per decade which brings it down to -20db at 10kHz, to -40db at 100kHz, etc.
You have to be a little careful though because sometimes the true curve does not follow the asymptotic approximation very well.
Maybe i'll post a Blog on this sometime.