Add SPF and PTR for hostname?

I'm using external nameservers for a Cpanel server, and I have server system e-mails/cronjobs forwarding to gmail, but (almost) everything ends up in the spam folder. What records can I add for the hostname in my external nameservers to reduce the chance of mail ending up in my spam folder? I'm figuring the SPF txt should be like this, and I can just add that to the external nameserver: v=spf1 +a +mx +ip4:serveripaddress ~all What should the PTR record look like? Is there anything else I can do? I've read that adding a DKIM record might not make sense.