Can you go into a bit more detail what you are trying to do? If you are setting this up through a script, I would suggesting using the metadata to grab the anchor IP.
You can grab it with
and it will return the anchor IP for the droplet. This would allow you to set this directly as opposed to setting a range that can vary based on region and other unpredictable factors.
If this doesn't work, please go into a bit more information on what exactly you are trying to do and perhaps a more specific solution can be offered.
Edited for errors.