Screenwriting : Writing contract question by Nicholas Klank

I've been asked to write a script for a best selling author but we haven't discussed a contract as of yet. I am not in the SGA and have never written anything but spec. Are there any resources for someone like me outside of the SGA? How do I ensure I am fairly compensated? What we've discussed so far is tiny in comparison to the SGA minimums. I appreciate that I'm working for an author and may not be regulated by the SGA. Thanks for your help!