SPF record clarification - Is this set correctly?

Question

I am not very familiar with SPF records so I need a bit of help setting up my SPF record correctly.

Below is a record I created using the Microsoft SPF record wizard

v=spf1 a mx ptr ip4:xxx.xxx.xxx.a ip4:xxx.xxx.xxx.b include:aspmx.googlemail.com include:mydomain.com -all

As you can see I use google apps, I also have two web servers sending mail on behalf of mydomain.com. I also listed two IPs both are web servers relating to mydomain.com and mail from mydomain.com are sent from both server (web app).

I also set the PTR to be xxx.xxx.xxx.a at my ISP. Considering all this and the fact that the above mentioned are the only places where mail get generated for mydomain.com is the above record correct?