Be aware that there is no single "safe" angle. Just Thursday I had 2800 lbs on the front end, loaded tires, 600lbs of 3ph carry all, was backing into and out of a shallow ditch (needed to set the concrete in bottom of the ditch).
At less than a 4% incline I got one rear wheel 1' off the ground. I set things down, repositioned things, picked up culvert again and placed it where I needed it. The only solution is to go slow, carry weight low, be ready to drop it.
I've learned it all depends on where the Center of Gravity is... and that depends on weight front, rear, height, angles of vehicle and all attachments, and whether CG is above or below axles. When backing straight out of the ditch, the engine went down, rear up, FEL down, CG moved above front axle and almost in front of it.... carrying FEL low was critical, load hit the ground and all stabalized. It happens very fast, even when moving very slow.