If they are using your licence number for the works, do you realise that YOU are responsable for the warranty as its your number, even after you leave the company. With that in mind I would have thought no less than $40 an hour
Unless you are the only plumber, you should be getting a percentage of all the work being done since you in effect are warranteeing it. And your warranty could extend for one or two years after the work is done even if you are no longer at the company. Also, if there is a dispute with a customer and it is unresolved your license could be suspended, if that is the procedure in your area. With these potential problems using your license, you should be negotiating a better deal than just hourly wages.