Correct way to set PTR DNS record for mail forwarding

I'd tried to set up mail forwarding from my Postfix server to Gmail. I'm getting the next error from Google server:

'The IP address sending this message does not have a PTR record setup, or the corresponding forward DNS entry does not match the sending IP.'

I've seen an instruction on the support page (section named 'PTR records') there:

https://support.squarespace.com/hc/en-us/articles/360002101888-Adding-custom-DNS-records-to-your-Squarespace-managed-domain

It advises to set up next values:

• In the Type dropdown menu, select PTR.
• In the Host field, enter @.
• From the TTL dropdown menu, select 1 hour. (What is it? There aren't any dropdowns.)
• In the Data field, enter data from your provider. (Which data? An IP address? A domain name? An IP address with '.in-addr.arpa'?)

I've tried to enter different combinations of values, like:

• Host: '@', Data: DOMAIN
• Host: '@', Data: IP
• Host: IP, Data: DOMAIN
• Host: IP.in-addr.arpa, Data: DOMAIN
• Host: DOMAIN, Data: IP
• Host: DOMAIN, Data: IP.in-addr.arpa

But the result was the same.

What values should I use to correctly set the PTR record?

A screenshot of PTR record edit dialog:

Same problem here. I need reverse resolution for my domain for setting up https via Let's Encrypt.

Have you found a solution?

No combination of '@', domain name, ip and ip.in-addr.arpa seem to work

Bumping this, because I am having the same issue. My emails are getting sent to spam, because I don't have reverse DNS setup.