The pool of IP addresses used on the internal private network is shared, so unfortunately there is not a range of addresses that you can safely assume will only be for Droplets created on your account. I’d suggest automating the process of spinning up the new servers using the DigitalOcean API. That way you can retrieve the private IP address and adjust your MySql permissions all in one go.

Another option might be to use a tool like Droplan which periodically queries the API and adjusts your IPtables firewall based on the private IPs of your Droplets. Check it out:

• How To Automatically Firewall DigitalOcean Private Network Interfaces with Droplan