I have a signed letter from a president of a very large company giving me a % for all future sales.

To have a contract, several elements need to be present, you need to contact a local attorney because it would take me longer than 4000 characters to explain it to you. I can tell you that Offer + Acceptance + Consideration = Contract (O+A+C=K)

Your question focuses on the form of the contract -- that it's in letter form, rather than some other. That's actually usually NOT very important.

I think what you really intend to ask is whether you have a legally binding agreement that you could enforce in court if necessary. Secondarily you want to know what damages you could recover in court for the other party's breach of the agreement. And finally, you want to know -- given the answers to the first two questions -- how to go about assessing a reasonable value to the rights you'd give up if you released your claims as part of a negotiated settlement.

The answers to these questions may be extremely fact-specific. The fact that there's been past performance under the letter, for example, is extremely important (and favorable for you).

But you need someone with whom you can discuss all the circumstances out of which the letter arose (including what you did or are doing to help them generate their sales) to help you assess how strong your claims would be in court and -- just as importantly -- what possible defenses the other party may assert. You also mention an on-going business relationship that appears to have some value to you, and that you're concerned about disrupting if you end up in litigation. Clearly, you need someone who can serve as your private counselor -- to help you evaluate all the angles before you declare war at the courthouse -- in addition to someone who'd be your competent and eager gladiator at the courthouse if things do come to that.

Nobody is going to be able to give you a set of competent, comprehensive answers to any of these questions over an internet service live Avvo.com, based just on the details you've provided here. And you probably ought not provide more details over any internet service, because even though you haven't used your name or any other identifying details, these are not private communications which can be protected by attorney-client privilege.

I strongly recommend that you consult with an attorney who's experienced in commercial litigation -- perhaps one whose practice is geared toward sophisticated individuals and small business owners. You may be able to find one who offers a free initial consultation, although that's frankly more common among personal injury lawyers than commercial litigation lawyers. Nevertheless, Avvo.com can be useful for finding and evaluating the credentials and experience of such lawyers.

Big law firms, of course, handle commercial disputes -- typically representing large companies -- all the time. I recommend, however, that you focus on the individual lawyer, not the firm, and make your choice based on who's actually going to be doing the legal work for you, not who's at the top of a pyramid of hourly-rate billers.

In theory, any lawyer with a Texas law license has taken the pertinent courses in law school and ought to be able to answer your questions. In practice, however, lawyers who don't handle commercial disputes like yours regularly will flail around and quite possibly make things worse through failure to identify and then pursue the most appropriate strategy for your specific situation. And lawyers who work for big law firms that tend to value only large corporate clients may be insufficiently sensitive to your needs (including your resources).

Good luck!