As to rulers, I already have the easy angle and companion angle, and use them for a lot of cutting.
I'm not sure about the folded corner one though. I've been cutting the squares, then lining them up on the cutting board to cut off the corner where the "covered corner" will go. The easy angle works perfect to cut the pieces that get sewn to the larger block. I don't worry about the dog ears, I trim them off as I go.
I'll check at my LQS (only have one) and see if they've got the corner ruler. The last two times I wanted rulers for a BH mystery - they had them. They didn't plan it that way . . .