The only generally applicable rule I have is that the founder relationship is ideally viewed as a long term and ideally repeatable relationship. If you leave anyone feeling screwed, it probably is a lose; you negotiate much harder and win-lose for a one time purchase with a stranger than with people you will be doing business with long term, especially people who need to be intrinsically motivated.

I think the same applies to employee salaries; negotiating someone down into taking a much lower salary than necessity is 10k sound and 10m foolish.