Thanks everyone for your helpful comments. It seems like there are few hard and fast rules to IP warming other than to slowly increase the volume of emails sent over a month, target an engaged audience and use an email optimised for deliverability.
On further research I found the Deliverability Implementation Guide and Deliverability Best Practices Guide which were very helpful. From reading these I see that you should aim to target an audience that is roughly equal to the number of emails you plan to send per month.
I also did some googling which suggested if you send less than 100,000 emails per month then you don't really need to worry about IP warming. Not sure what other peoples thoughts are on this. Our database is definitely below this number but I will certainly be going ahead with IP warming.
