From MiLacnic management portal delegation can be set for /16 or /24.
To set custom nameservers for rDNS delegation at LACNIC follow the steps below:
Login to MiLacnic portal at https://milacnic.lacnic.net/lacnic/login
In the menu on the left click IP/ASN to manage resources:
Click Delegate rDNS in a master prefix dashboard:
In the list, find block for management and click Delegate button to create delegation or Edit to alter
Set Primary to ns8.heficed.com, alternate to ns9.heficed.com and click "Next" by completing all steps: