Most providers today are either just starting with a /20 or have grown to somewhere around a /13 in total. This chart goes up in /20 increments from /20 to half a million IP addresses which is just short of /13 by about 20,000 addresses.
Most impact of the HD Ratio is felt between the /20 and /13 sizes.
The top of the orange bar is important at all sizes because this is the threshold that your total allocation must reach.
The top of the green bar is only important up to around the /15 point because very few ISPs get a new allocation bigger than that size.