I don't think the relationship should be linear - 2.5x the work for 2.5x the hours...
If you count waking hours, a typical (3 weeks vacation M-F 9-5) job expects 49 *40 / (52 * 7 * 16 ) = 1960 / 5824 = right at 33% of your life. As they take away my free time, I have to hire (and manage) people to do things for me that I would normally do for myself - this drives up costs by as much as 3-4x in many instances, and can become incalculable when it means hiring people to watch my children grow up for me.
I think a fair overtime formula would be paying you 4x your standard hourly rate by the time you reach 80 hours a week, something like P = P * (H / 40)^2 when H > 40 - by the time they've taken all your waking hours, pay would be at 9x standard. Is getting the project out on-time critically important to the financial future of the company? If it really is, the team that "gives their all" should be getting more than "attaboys" in return.
If it's a short-term crisis where 4 or 40 extra hours worked at a critical juncture can be "paid back" in comp-time within a week or two, that makes total sense in an "overtime exempt" relationship. If the crisis is dragging for months and all you're getting is a gold star on your annual review, put in a few extra hours a week to shop for a better working relationship.