What is the (most) correct way of naming droplets and plan ahead?

Taking into consideration that one day I’ll have more than 3 droplets (one for DBs, mail, www, etc), that I want to have vanity nameservers, correct PTR records, etc, etc. How should I name my droplets so it’s easy to understand and won’t create problems?

For the sake of example, here is my domain : domain.tld.

I create my first droplet, and add domain.tld as it’s hostname, creating in the same time it’s PTR records in the DNS management. I then add a domain name domain.tld in the DNS management. I then proceed to add vanity NS infos and glue that to my registar. Hurray, I’ve got a basic website.

Now, I want think about the future and name things correctly. The machine I’ve just created will be used for websites. So I’d like it to be “www.domain.tld”, Do I just add an A record for www.domain.tld and change the hostname of the machine?

After that, I’d like to get my DB server running… So I create a machine with hostname db.domain.tld and change DNS records accordingly.

Now a mail server. Creating mail.domain.tld and changing DNS… Do I need to update PTR records?

I guess a tl:dr version of all of this needed, so here it is: what do I have to take into consideration while naming droplets if I want to think “big” and plan to have dedicated droplet for future growth? How should I name things? What do I have to change?