rDNS and I'm confused

I suppose I'm not wired to understand dns... it sort of blows my mind.
Anyway although I can see a lot of topics covering this I cannot for the life of me understand what I should be doing to correct the problem. I'm running CWP7 pro for several domain. I've set up freedns manager and all the domains seem to be working fine.
My emails are being returned as the rDNS doesn't point to my server.
Now I know that I have to correct this at the ISP end and they do have a panel for me to change the rNDS and that's my problem. I have no idea what to change it to.
At the moment the rDNS hostname is set to xx.xx.xx.xxx.dsl.in-addr.zen.co.uk.
(xx being my ip address) I'm guessing that I should change the end (.dsl.in-addr.zen.co.uk.) to reflect my server, so where in CWP would I find that information please.

You can setup rDNS/PTR record as your domain name or server hostname.

For ex:- 

192.168.1.1 -> domainname.com
192.165.1.1 -> server.domain.com.

Check this link for more details:- https://blog.leadfeeder.com/what-is-reverse-dns-and-why-you-should-care/

Thanks I thought I did that.
I set the server host name to server.mydomain.co.uk when I installed CWP and then created an account for mydomain.co.uk. As I'm using freedns manager I setup the dns through that for mydomain.co.uk. I then setup an "A" record within the dns for mydomain.co.uk for server and pointed that to the external static ip. I just assumed that I had to change the PTR records at my ISP to serverip and my hostname... but nope didn't work.

you only need to have valid A record for your hostname and PTR set by your hosting provider to match the hostname.

- Add an A record to your domain, same as your hostname, point it to your server's IP
- Go to your hosting provider dashboard, find rDNS and put your hostname there