The basic scenario is that my former employer has been approached by another company who wish to license some software I wrote for them while I was in their employ and have approached me to work with them on making the changes required to commercialise this software (it is currently very specific to my former employer's environment and systems, so some work is required to change this).
My former employer is not a software company and has no software developers of their own so this is strictly a side-project for them as well.
I am currently in full time work elsewhere so anything I do will be in my own time at evenings or weekends. They have suggested a commission deal whereby I get a commission on every sale, which I am fine with. I expect there will be a maximum of 100 hours of work required prior to first sale.
We have still to work out details of who supports the customers, I certainly have no intention of being first line support!
My question is how do I go about working out what sort of commission is reasonable in this scenario? What factors need to be taken in to account?