Clients, contracts, and employers, who owns the rights and how should you charge

I wanted to start an open discussion about how to deal with clients and how you structure different contracts. 

I have my own company that provides designs services to the automotive and product design market. Since Algorithmic Modelling is a new type of service many of my clients have no idea what they own and don't own. I have my own understanding and I every contract is different so I wanted to ask what is common practice.

I work as an outside consultant who sometimes works inside of studios for modeling jobs. Many of these I have to write algorithms for. I use my own licence and register all my codes to my company.

In my contracts, the deliverable are final design surfaces, so that is all they keep. 

Some people might get the impression that they now own that algorithm and I can't use it for another client. Even if it's a very simple algorithm, it's like they think a hexagonal pattern that changes is innovative. I wanted to know what is common practice in these types of agreements. Right now my contracts are easy and straightforward. You want this design, in this package, I provide you the model. 

What if a company wants to buy my algorithm so they can keep using it over and over again. Can I charge a licence fee so that as long as they are still using it I get paid for it. Can I even do that if I am using another companies plug ins and components. Does that mean I am giving them the rights so I can't sell my own algorithm anymore?

Is it better to charge more money for them to keep the algorithm or to establish a ongoing relationship. Where I keep the algorithm and whenever they want to use it again, I charge them the service? 

It is my understanding that Grasshopper is just an extension of my skill sets, most of the things you need to do for automotive and product design are quite simple and aren't complex enough to have rights to a design.

I may be completely wrong in all this, I am a self taught person. I  don't know any others who are doing what my company does so I am sort of shooting from the hip here. Keeping a similar contract to when I provide a modeling service. Any advice would be much appreciated. I am also very aware that a contract can be whatever two business entities agree upon, what I am wondering is what is common practice and the "correct" way to approach this. 

My website is Handlebar3d.com for those who are interested. Thanks for taking the time to read this.