adeguilo:
As others have said, angle can vary on conditions. It's best not to go on a number (i.e. a particular number of degrees) because conditions alter that - also, as another poster well explained dynamic factors (momentum) changes the extact angle when point of no return is going to happen.
I have gone over twice. Once with a farmall coming up out of a hole I was digging with a full scoop of wet dirt. The farmall is pretty light on the front end (blush, blush. I sometimes drove the thing on the two rear wheels while the front wheels were off the ground - fun, but NOT a very safe practice). Fortunately, the 3pt kept damage to a minimum (just the top link bent). Harry Ferguson (developer of 3pt) and Henry Ford (who first popularized it on the 9n) saved a lot of lives. Maybe saved mine. Also, went over forward in a skid steer with little risk. Load of dirt too high when dumping while moving forward - the ANGLE was safe enough, it was the high center of gravity combined with the forward momentum that caused it. Again, you can't go strictly on a specific angle - you have to consider conditions. The cage structure protects well (unless you are thrown forward out of the cage). The bucket kept it from a complete 180. No damage (I guess - it was a rental). Easy enough (when got over the shock!) to push it back upright using the bucket.
Am more afraid of going over side ways (unless I was in a skid steer) which is why I won't wear seat belts. If the thing starts to go over, you can leap off to the UPSIDE - IF you are paying attention. Also, there's nothing to stop the momentum side to side once it starts to roll. When going over fore and aft the rolling is stopped (usually) with the upper link in the rear if you go over backwards or whatever implement is on the front - usually the FEL - if you go over forward ).
As others have said, when these things happen they happen FAST unless you are totally aware of what you are doing at the time. Paying attention is the single most important safey neccessity. Example: you are heading uphill at an obligue angle on a slope that seems steep. The upside front tire will tend to lift a bit first when you are getting into a risky situation. If you see that, whoa. My advice: if anything even looks risky either don't do it, or, go very, very slow and PAY ATTENTION to what is happening with the tractor.
JEH