I want to configure SPF on a domain to allow mail to be sent from:

• the x.com website server (x.com and www.x.com - both at same IP)
• it's MX servers (smtp.x.com, mx.x.com, mail.x.com)
• another server that isn't listed as an MX server (somehost.x.com)
• via gmail using an account that has authenticated use of [email protected]

Will this zone file work? If not, what are the problems with it?

$ttl 38400
@           IN      SOA     ns1.x.com. hostmaster.x.com.  (
                                      201003092 ; serial
                                      8H    ; refresh
                                      15M   ; retry
                                      1W    ; expire
                                      1H )  ; minimum
@           NS  ns1.x.com.
@           NS  ns2.x.com.

@           MX  10 mx.x.com.
@           MX  20 smtp.x.com.
@           MX  30 mailhost.x.com.

; SPF records
@          IN      TXT "v=spf1 a mx a:somehost.x.com include:_spf.google.com ~all"
mx         IN      TXT "v=spf1 a -all"
smtp       IN      TXT "v=spf1 a -all"
mailhost   IN      TXT "v=spf1 a -all"

Questions:

1. Is _spf.google.com the right thing to include for gmail.com, or is it only for Google Hosted Apps? If only for Google Apps, what should I include to send from gmail.com?
2. If mail shouldn't be sent from anywhere else, is it safe to use -all instead of ~all?
3. Does it make sense to add specific SPF records for each of the mail servers?
4. Any other problems with the zone file?

I want to confirm these things before making changes to my zone file. The file has SPF configured basically the same now, just without google.com and somehost, but I want to make sure I won't break things when I change it.