Not sure what you really need to know for. There about a million variables that could factor into it. If it's just for bragging rights then get yourself one of those little clinometers with the bubble and have at it. When you feel your pants get wet that's probably far enough.
As a general rule of thumb when on the trails I like to keep my rig pointed strait up or down...It's not really a great feeling to have your rig sliding sideways down a hill. And remember if you feel like you’re getting in trouble point it down slope and gas it.