The MX record for a domain specifies the mailservers which receive email sent to that domain. Those servers may or may not also send email for that domain. If they do, you will need to indicate this in SPF by including "mx" in the SPF data. Assuming the A record also sends email, the SPF data would look something like this.

In this case the servers listed in the MX record are checked in order of preference, and if the IP for any of those servers matches the sending mailserver's IP, the SPF check results in a match. If you want to specify a group of servers based on the same IP as those in your MX record, you can include the CIDR range after the mechanism, like "mx/27."

You can also of course specify another domain or host, and the MX records of that domain will be checked and considered valid to send email for the listed domain. This would again take the form mx:otherdomain.com, with or without a CIDR block as appropriate. So the SPF data for a domain sending through another domain's mailservers might look something like the below.