Tracking Tune Ups and Conversion To Service Agreements

I am working on the best way to track how many tune ups we are selling and converting those tune ups to service agreement contracts. I am looking for any input on how everyone is accomplishing this. I was thinking of using a specific billing code such as PTU (precision tune up) to let me run a sales item report on how often it was sold and not sure what the best way to track the agreement conversion off of this would be. Maybe i should be doing this through dispatch types instead? any suggestions would be much appreciated. Thanks!