As far as I understand, in order the same logic to be utilized you should replace SrvRipSaw1.ProcessingTime expression with Server1.Processing.Contents.AverageTimeWaiting. Note that each time it is referenced the expression SrvRipSaw1.ProcessingTime samples one observation from previously defined expression in ProcessingTime property of SrvRipSaw1 server. On the other hand, Server1.Processing.Contents.AverageTimeWaiting returns the overall average of all processing times that has occured (completed) till at the moment. There seems no poroblem with your WIP expression.
Note: It should be noted that SrvRipSaw1.InputBuffer.Contents.AverageTimeWaiting + Server1.Processing.Contents.AverageTimeWaiting returns "average cycle time" of SrvRipSaw1 server.