Although I prefer regular meetings, communication and updates as a method for informing the client of the project status; my client wants an overall RAG status for each of their projects. I was looking at an objective way of combining risks, issues and progress against milestones to do this. What method have/would people use(d) to do this?
The thing to remember about RAG status indicators is that 1) they're not objective, 2) they're different for every project, and 3) they're different for every company.
Before you set up the status report you need to find out what those indicators mean to your client. For some clients a 10% deviation from plan is Amber, and others it may be Red. It's going to be based quite a bit on their particular risk tolerance. The more significant the project is in terms of how it fits into their company, the more risk averse they're going to be, so they're indicators will trigger sooner.
As the PM, you're going to need to discuss this with them and find their tolerance levels to set up the status basis. From there, as you approach a change in status you'll need to keep their view in mind, and temper it with your experience to reassure and inform them.
We don't do RAG status updates for our stakeholders, but I have a Kanbanboard setup in Jira (attlasian calls this a rapid view) which tracks only issues tagged with "project_board". That way, stakeholder, not interested in the details, can have a simple highlevel overview of the project status.
For our team, we do storypoint estimation for tasks. If we compare our velocity whith the remaining storypoints and days, we have a good guess about being able to fullfill our commitment.
If I had to do a RAG report, I would base this on the storypoints and velocity. This covers your progress and issues somewhat, but I have no idea how to fold "risk" into the equation, sorry.
I think you can set RAG status for budget, schedule and scope. As already Trevor mentioned, agree with your client on a clear definition of your ranges. Also, take a look at the resolution of your status indicator: per project, per phase, per milestone...
Take into consideration that by applying such status indicator, you will need to be very strict on measuring the variables, otherwise you will end up with discussions like: "Is it amber or red? Can we say... orange?"