I'm actually not in complete agreement with aag, as this formula isn't correct if you're applying it to a cluster compute resource.
What you're trying to do is simply convert the Mhz value of a metric at the cluster level to Ghz, right? A simple /1000.
You simply want to do this formula. "$This:CPU Usage|Demand(MHz)/1000". Make sure you use the Attribute ID instead of the Metric ID.. so your actual formula will look like $This:A12345/1000.
No sum, no sumN... that's all unnecessary in this case. Use $This instead of$R or $RK since you're generating a SM from the values only from the resource upon which the SM is applied.