If your server is foo.com and your hosted domains use the foo.com mail server, then the text line in your default DNS would read:
<domain>. TXT v=spf1 a mx ptr include:foo.com ~all
and the DNS for all hosted domains would have a corresponding TXT DNS entry:
hostedSite.com TXT v=spf1 a mx ptr include:foo.com ~all
You should add it to the default DNS in the Server CP and then manually apply it to the DNS record of currently hosted domains.