I looked for about an hour googling "mercury 9.9 prop nut torque". None of the recommendations were less than 55 lb-ft so all recommendations were worthless. The nut is bronze and small in diameter so 55 lb-ft could strip the nut. I'm probably going to use 10-15 lb-ft and call it a day.
You can also purchase a proper prop wrench which will not allow over tightening. They are made of plastic and can be purchased at any marina and likely on line as well? (Amazon)
Truthfully, I have never used a torque wrench on mine. I just tighten it until it is snug but not so tight that I can't move my prop in and out a few millimeters. Once I am there, I bent the lock tabs over and call it a day. It is also a nylock nut so I have no worried about it ever coming off.